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What Age Does Old Person Smell Start? What the Studies Say

Every thread on this subject turns into the same argument: one group says it starts at 40, another says they have never smelled it on anyone under 70. Both are quoting real evidence. They are quoting two different kinds of evidence, and the gap between them is the whole answer.

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Few numbers in consumer health get repeated as confidently as this one. Age 40 is where the stale, greasy smell is supposed to begin, and the figure turns up in product copy, news write-ups and the first line of almost every forum thread on the subject. Then the thread fills with people saying they have never once smelled it on anyone under 70, and that the whole thing sounds like an advertisement. A discussion on r/todayilearned this morning ran to a hundred comments and spent most of them on precisely that disagreement.

Both sides are quoting real evidence. The reason they cannot settle it is that the question has two answers, produced by two completely different kinds of measurement, roughly thirty years apart.

Where the number 40 actually comes from

The whole age-40 claim traces to one paper: Shinichiro Haze and colleagues, published in the Journal of Investigative Dermatology in 2001. They collected body odour from subjects aged 26 to 75 and analysed it by headspace gas chromatography and mass spectrometry. One compound stood out, an unsaturated aldehyde they described as having an “unpleasant greasy and grassy odor”, and the sentence that launched a product category reads: 2-nonenal was “detected only in older subjects (40 y or older)”.

Three things about that sentence matter and are almost always dropped.

The first is the sample. Twenty-two people spanning fifty years of age is a small study, and the commenters who keep flagging that are right to.

The second is that the authors all worked at Shiseido, which went on to sell products aimed at this odour. That is not a reason to discard the finding — the mechanism has held up, and later independent groups measured the same compound rising with age — but it is a reason to label the source, which we do throughout our index of the nonenal research.

The third is the most important and the least discussed. “Detected” is a word about an instrument. A gas chromatograph was able to find the compound in the headspace above the skin of people over 40. Nothing in the paper claims that a person standing next to them could smell it, and nothing in it claims that the smell would be identified as old age. The number everyone quotes is a detection threshold for a machine.

What happens when you ask noses instead of machines

There is exactly one published experiment that put the perceptual question directly, and it is the study missing from every argument about this. In 2012 a team at the Monell Chemical Senses Center in Philadelphia collected body odour samples from three sharply separated age groups — Young, 20 to 30; Middle-age, 45 to 55; and Old-age, 75 to 95 — and had 41 young participants rate and try to identify them.

People could tell the difference. Participants discriminated between the age categories, and the effect held up even after statistically removing the influence of intensity differences. But the ability was lopsided: participants “were able to correctly assign age labels to body odors originating from Old-age donors but not to body odors originating from other age groups”. Middle age, in other words, did not smell identifiably like middle age.

Then there is the finding nobody expects. Body odours from the oldest group were rated as less intense and less unpleasant than those from both the young and the middle-aged donors. The most unpleasant samples in the experiment did not come from the 75-to-95 group at all. That result, and what it means for a smell everyone assumes is simply bad, is the subject of our guide to what nonenal smells like.

So the perceptual answer to “what age does it start” is somewhere around the mid-seventies, it is about being recognisable rather than being offensive, and on the pleasantness measure the age band most people are anxious about scored worse than the one they are afraid of becoming.

Why both camps in the argument are correct

Line the two findings up and the forum argument dissolves. An instrument can pick up 2-nonenal in the air around someone from about 40; other people begin reliably labelling a body odour as old age around 75. Those are not contradictory claims but measurements of different things, and the ordinary meaning of “starts” sits awkwardly across both.

A 2016 Japanese group added the piece that explains why personal experience varies so much within a single age band. Using a passive flux sampler pressed against the skin, they found emission flux of 2-nonenal increased with age in both male and female volunteers — and that emission “remarkably varied with sampling position”, enough that they had to fix the nape of the neck as a standard site. Where on a body you happen to get close to someone changes what you smell.

That observation, repeated by many people in that thread, is not scepticism about the science. It is a fairly precise restatement of what the perception experiment found.

The precursor story gets stranger after 80

If you want to know when the smell starts, the obvious place to look is the raw material. Haze’s 2001 paper reported that omega-7 unsaturated fatty acids and lipid peroxides in skin surface lipids both increased with aging, correlating with 2-nonenal — a tidy story where more precursor means more smell.

A 2003 study from the University of Tokyo measured skin surface lipids in a much older group and found the opposite. Free fatty acids and squalene were significantly lower in the older cohort, averaging 83.7 years, than in a group averaging 22.2 years. Linoleic acid composition fell by 40 per cent, and the monounsaturated acids dropped significantly too — including palmitoleic acid, which the authors note is precisely the fatty acid whose oxidation produces 2-nonenal.

The two are not flatly in conflict, because they sampled different parts of life: Haze’s older group started at 40, the Tokyo group averaged 83. Together they suggest the precursor supply rises into middle age and falls away in advanced old age. Work on post-menopausal skin points the same way, with sebum excretion declining significantly over the first decade after menopause — oil output going down over exactly the period in which people report the stale smell going up, which only makes sense if the problem is oxidation rather than quantity. That is a point we lean on in why the soap sometimes seems to stop working.

The plain conclusion is that nobody has published the age curve, and anyone offering a start date more precise than “detected from about 40, recognised by others much later” is going past the evidence.

Does everyone get it, and is it genetic

Nobody knows, and the reason is structural: every measurement in this field reports group averages from small samples, and none followed individuals over years. A study can tell you that average emission rises with age. It cannot tell you whether you personally will end up with a smell other people notice.

The genetics question arrives in every thread via the gene that stops underarm odour. A variant in ABCC11 reduces the apocrine-derived precursors behind classic body odour and is common in East Asian populations, and people reasonably ask whether it protects against this smell too. It almost certainly does not, because it is the wrong pathway. Underarm odour is made by bacteria working on apocrine sweat; 2-nonenal is made when skin oils oxidise, with no microbe in the mechanism at all. A title-and-abstract search of PubMed for ABCC11 and 2-nonenal together returns nothing, so the question has never been studied directly.

And the gene is weaker than its reputation even in its own lane. A 2014 Unilever study of the variant found axillary odour significantly lower in people carrying two copies — but noted that “the perception-based measures still detected appreciable levels of axillary odour in the A/A subjects”. A low-odour genotype is not an absence of odour.

The activity theory, and what nobody has measured

The most upvoted explanation in every one of these threads is lifestyle: active people do not get it, and it is really about not showering, not changing the bedding, not keeping up with the laundry. One commenter who had worked maintenance in a care home put that case bluntly, and others replied with grandparents who stayed active into their nineties and smelled of nothing.

This theory has never been tested. Walking all ten published papers that mention both 2-nonenal and body odour, the only intervention study among them is a 2022 trial of blackcurrant powder in 14 adults. Not one study has measured this compound against activity level, bathing frequency or laundry habits.

What is established is the mechanism, and it makes the laundry half of the theory much more plausible than the exercise half. The compound is generated by oxidation of oils that are already sitting on skin and in the fabric that skin touches; it is strongly oil-soluble, with an XLogP3 of 3.1, so it does not leave in water; and once formed it becomes covalently bound to protein, chiefly at lysine residues. Polyester both holds more bacteria and absorbs more sebum than cotton, which is why synthetic bedding and upholstery behave as reservoirs that keep producing smell after the person is clean. That is why we treat the chair and the sofa and the wash cycle as separate problems from the shower.

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Why the people best placed to tell you will not

There is a final reason the age reports in these threads are unreliable in both directions, and it cuts against the confident ones hardest. The ability to smell declines steeply with age. A population study of 2,491 adults aged 53 to 97 found impaired olfaction in 24.5 per cent overall, rising to 62.5 per cent of those aged 80 to 97 — and among affected people in that oldest band, self-report caught only around 12 per cent of women and 18 per cent of men.

So when someone in their sixties reports that neither they nor their eighty-year-old relatives have this smell, two things could be true, and one of them is that the detectors are failing. The poster who started the r/AskWomenOver60 thread put the question in a form that contains its own trap.

One of the first replies pointed out, gently, that she might want to be careful about assuming she and her husband are in the clear. That is why we keep recommending a fabric check over a self-sniff: our guide to whether you can smell your own body odour goes through what the check can and cannot tell you.

What to do with a number like 40

Treat it as what it is: the age at which a laboratory instrument first picked up a compound in a small Japanese study in 2001. It is not a deadline, and it does not predict that anyone will notice anything about you.

Something more useful than an age is available for free. Wash a pillowcase, sleep on it over a clean or protected pillow, and smell it first thing before you open a window. That is a real test of a real reservoir. If it comes back stale, the laundry and the bedding deserve as much attention as the soap. If you are weighing bars, our comparison of the best persimmon soaps sets out what each brand does and does not claim, and what the evidence on persimmon soap actually shows is blunter about the gap than most pages: there is no published trial of persimmon soap on human body odour at all.

The bottom line

Nobody is lying in these arguments. A machine can detect 2-nonenal in the air around people from about 40; the one perception experiment says other humans reliably recognise the smell of old age only from around 75, and rate it as milder and less unpleasant than middle-aged body odour. The precursor data does not agree with itself across age ranges, nobody has published the lifetime curve, and the most repeated explanation online — that it is really about activity and washing — has never been measured.

What survives is modest and practical. The smell forms in skin oil, so it accumulates wherever that oil goes, which makes fabric both the best place to check for it and the best place to deal with it. An age on a calendar tells you almost nothing. A pillowcase tells you something.

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