Fair point. The cannabis example is a good one, that has been true for myself. I always thought it was more to do with smoking regularity and being out of practice, or maybe less carefree. I used to smoke a fair bit, then stopped for a long period, got older, and now I just can't smoke very much. But it could definitely be due to an actual change in brain chemistry/function.
I've never tripped with a 70 year old, but I have tripped (lsd) with a 50 year old and it seemed to work just the same as it did on me, as a 25 years old.