I think a lot of what goes into parents' decisions to not trip again is that they feel they've gotten what they need from the drugs and the experiences, ie.: humanity's place in the universe, the extent of the cosmos and the sense of micro and macro that implies, the infinite complexity and beauty of nature, etc. I share that feeling to a certain extent and I could be the parent of some of the younger BLers.
I most certainly have not ruled out tripping again. In fact, I plan to. But I have some emotional shit that I need to sort through before I feel that I can trip again without having to unpack and examine a shipping crate's worth of baggage. Maybe that's part of what keeps previous trippers from stepping off into the cosmos again.
I know that when I was 17-23, in active, mature tripping years, I would have said, "That's more reason to trip then. Sort it out. Pull it out. Unpack it. Examine it. And let it examine you." But I have unpacked. I have unsorted. I have looked at a lot of my conflicts eye-to-eye.
But there are other things that I've experienced, am experiencing and will experience that I don't think would be aided by tripping. For example, the current health problems and impending deaths of my mother and father. I was adopted by parents who were much older than my peers' parents (not only conservative, but actively looking backward: refused to rent even a VHS tape. You see what I'm saying?). So I have been dealing with their decline and eventual death for a number of years now (cancers, strokes). I am their only child (and adopted as I mentioned) and I have chosen to support them until death even though they were/are pretty damn abusive (obv. they can't get away with the physical shit now). Getting away from them would be very healthy for me. But I made my choice.
I feel it's issues like these that keep many older ex-trippers decline to trip again, at least for the short term. Life is difficult in many different ways at different ages. I think even some experienced psychonauts (I'm not speaking of me here, but rather of a friend who's a highly experienced uber-tripper) feel that they have enough to deal with in life right now that they need every second just to deal with reality as it presents itself. And the situations they're facing wouldn't benefit from further examination.
I think that's why you see so many people in their 60-70s go back to tripping. (I, at least, have seen it a lot.) They have eased their parents out of life, their kids are grown, and they're free enough to unpack some shit and see where they are after the intense mid-life ride.
Obviously, I haven't described everyone. I know several 50 y.o.'s who trip their nuts off every few months. But I have described what I've seen in several large groups of friends and acquaintances. Just some thoughts…