Old person smell is the characteristic odor of elderly humans. Much like many animal species, human odor undergoes distinct stages based on chemical changes initiated through the ageing process. Research suggests that this enables humans to determine the suitability of potential partners based on age, in addition to other factors. "There were significant differences in ratings of both intensity and pleasantness, where body odors from the Old-age group were rated as less intense and less unpleasant than body odors originating from Young and Middle-age donors."
"[A]n unsaturated aldehyde with an unpleasant greasy and grassy odor, was detected only in older subjects (40 years or older). Another study failed to detect 2-nonenal at all, but found significantly increased concentrations of benzothiazole, dimethylsulphone, and nonanal on older subjects.
In 2012 the Monell Chemical Senses Center published a press release claiming that the human ability to identify information such as age, illness, and genetic suitability from odor is responsible for the distinctive "old man smell". Sensory neuroscientist Johan Lundström stated, "Elderly people have a discernible underarm odor that younger people consider to be fairly neutral and not very unpleasant."
Old person smell is known as kareishū (加齢臭) in Japan, where it is of particular concern due to the high value placed on personal hygiene.