There was something about exactly that topic on NPR the other day - older guys doing meth. The explanation they gave is that a lot of guys who came of age in the early days of AIDS had lived relatively ascetic lives and felt like they had missed out on a lot and so they were experimenting with PnP.
People seem to think that PnP is a relatively new thing, but in the 70s, when I was in my mid-twenties, PnP was extremely common. The big party drug then was MDA. I remember, even at that robust age, I felt like hell for days after a night of MDA.
Now and again, the opportunity to PnP comes up and I'm always tempted, but I remind myself how shitty I felt the morning after and that it would probably be 5 times as bad now in my 60s.