I don’t think you can lump all old people into one category that implies that they all have an “old person smell”. There are definitely some that do have a distinct scent, although these smells are not necessarily similar from person to person. With that being said, there are some people who are young who do not smell great.
This can be associated with poor hygiene. If a person doesn’t brush their teeth, wash their hair often enough, wear deodorant, or bathe regularly, all of these factors can contribute to the smell they will give off. If somebody is elderly and bedridden, this may be more likely to happen if they are not cared for properly. It is much harder to keep a person as clean as they should be if they are not mobile, although it can be done.
I know many elderly people who smell clean, or who actually have no apparent smell at all. Some seniors like to wear a specific perfume or cologne. Sometimes, they may not realize how much they have used and this scent can be overpowering. I think that this can happen somewhat more frequently with elderly people as their sense of smell may have diminished with age.
Some people like to use common scents – my father used to use “Brut” aftershave. He used it for many years, but I discovered that many other men used it as well. Every time I smelled that particular scent, I thought of my father!