Here's a weird thing: a study in 2012 found that humans are able to tell whether someone is young or old based on smell alone.
In the study, researchers collected the body odors of different age groups by asking them to wear T-shirts in bed for five nights with underarm pads to soak up their sweat. These pads were then cut up and placed into jars, which were sniffed by lucky volunteers aged between 20 and 30 years old, who were then asked to rate the pleasantness of the smell and estimate the donor's age.
"This experiment suggests that, akin to other animals, humans are able to discriminate age based on body odor, alone, and that this effect is mediated mainly by body odors emitted by individuals of old age," the team wrote in their study, adding that "the mechanism behind this effect is not currently known, even in non-human animals".
"Elderly people have a discernible underarm odor that younger people consider to be fairly neutral and not very unpleasant," sensory neuroscientist and senior author on the paper, Johan Lundström said in a statement. "This was surprising given the popular conception of old age odor as disagreeable. However, it is possible that other sources of body odors, such as skin or breath, may have different qualities."
A separate study found that odor-producing component 2-Nonenal, "an unsaturated aldehyde with an unpleasant greasy and grassy odor," as well as certain lipids on the skin, increase as people age, suggesting these have a role in that distinct smell.
As for the purpose of producing and being able to detect these smells, Lundström's team tentatively suggests it may be to do with finding suitable mates. The team notes that older male insects are preferable to younger insects, with higher reproductive success, likely as it shows they have adaptive qualities which allowed them to live to old age.
"Similar to other animals," Lundström added, "humans can extract signals from body odors that allow us to identify biological age, avoid sick individuals, pick a suitable partner, and distinguish kin from non-kin."
More study is needed to figure out the exact cause of the smell, and what advantage it might have for humans.
The main paper discussed was published in the journal PLOS One.