Psychedelic-induced plasticity (ability to re-route) seems to lend itself to folks retaining their cognitive function in spite of damage (e.g. in context age, stroke).
Reference? A lot of the after-effects of severe stroke are not unlike those induced by psychedelics (from personal experience of the latter days of my grandfather).
It didn't seem to be doing Alexander Shulgin any favors in old age, from what I saw of his later interviews anyways. Though he probably exposed himself to far more than just psychedelics.
Shulgin did suffer a stroke and dementia in his old age. His wife, who probably did just as many (and many of the same) substances he did survived him and is doing fine.
Something else to consider is that Shulgin said he usually only ever did maybe two or three sessions with any given substance, and then moved on to another one. It could be that one or more of them was beneficial, but he might not have taken enough of those particular ones to help. In contrast, Stanislav Grof had over 100 LSD sessions.
I don't think enough is known about the relationship between psychedelics and neurodegenerative diseases to say how and which psychedelics affect them, but there are some interesting ongoing studies to see what effect microdosing LSD has on Alzheimer's disease, the Phase 1 trials of which have recently completed:
It's not like I have them bookmarked or something.
Many years ago after reading PIHKAL I went a bit down this rabbit hole watching a bunch of what's on Youtube associated with Shulgin. He had given some presentation where he was clearly experiencing some mental problems. I'm sure you can find examples if you go digging, I don't have time to do that for you.
Sorry, but your critical claims of Shulgin requires strong evidence. I'm willing to believe he struggled in his old age, but need to be convinced that it was due to psychedelic use and not post-stroke and normal dementia.