Older folks give off a characteristic scent that's independent of race, creed, or diet. The Japanese even have a name for it: kareishu. Most people say they find the smell disagreeable, typically describing it as "stinky-sweet." But in a new study, participants in a "blind sniff test" found the body odor of older people less intense and more pleasant than that of the young or middle-aged.
Sensory neuroscientist Johan Lundström has been familiar with old-person scent since his childhood in Sweden, where he sometimes accompanied his mother to her job at a nursing home. Decades later, as the head of his own lab at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, he gave a talk at another nursing home. "The same smell hit me again," he says. Lundström wondered if there really are specific age-related odors that the human sense of smell can detect. Although research shows that animals can distinguish the ages of other animals based on their odor, no comparable studies had been done in humans.
So Lundström and colleagues recruited 20 men and 21 women between the ages of 20 and 30 to be sniffers. All were healthy nonsmokers who didn't take drugs or medications. Meanwhile, a group of "donors" who were young (20 to 30 years old), middle age (45 to 55 years old), and old (75 to 95 years old) went to bed for five consecutive nights wearing T-shirts with absorbent pads sewn into the armpits. To make sure they gave off only their natural scent, the donors washed their hair and bodies with odorless shampoos and soap before going to bed each night. They also refrained from smoking, drinking alcohol, or eating spicy food.
The volunteers sniffed the pads worn by the variously aged donors and grouped the smells by age. They classified the smells of the older donors with 12% greater accuracy than would be expected by chance, compared with 8% better than chance for the younger and middle-aged donors , the researchers report online today in PLoS ONE.
According to Lundström, the real surprise came when the sniffers were asked to rate the smells by intensity and unpleasantness. Even though the volunteers compared the smell of old people to stale water or old basements, when they encountered the smell amid those of the other age groups, they consistently rated the old person odor as the least intense and least unpleasant of the three.
Lundström says that people who find the elderly smell unpleasant may be setting it in an unattractive context, like a dreary nursing home or a stuffy parlor. "Context is an important part of the human sense of smell," he notes. "Many people think parmesan cheese smells like vomit if they don't realize what it is."
The researchers also found that when the "scented" pads were divided by gender, the sniffers deemed the smell of young and middle-aged males to be the most intense and unpleasant—describing it as musky, "horsey," or simply sweaty. By contrast, the smell of the older men was rated slightly more pleasant than that of the older women. Lundström surmises that as men get older, the drop in testosterone levels causes them to smell more like women and, presumably, less offensive. 
The ability to sniff out someone's age may have conferred an evolutionary advantage, Lundström says. "When the human sense of smell was evolving, people didn't live as long." It's possible, he says, that those who lived longer were assumed to be stronger, healthier, or smarter and would have children who would be better equipped to survive. Thus they'd be seen—or smelled—as the most desirable mates.
The fact that humans have some ability to discriminate age based on body odor is provocative, but the reason remains unclear, says neurologist Jay Gottfried of Northwestern University Feinberg School of Medicine in Chicago, Illinois. "There may be intrinsic differences in the odor compounds produced by each age group, or it could be that older people simply sweat less."