It turns out that the “old people smell” is not (necessarily…) a result of poor hygiene, but is caused by 2-nonenal, an unsaturated aldehyde, which is produced when certain skin oils oxidise and break down. More details are available here and elsewhere. The chemical is also found in aged beer, apparently…
(The featured image is a bit dodgy… It was just a quick creation using Gemini, but seems to have one pentavalent carbon atom…)