The sport was less physically taxing in the 60’s and before, so you had a lot of players continue to be good into their 30’s. So Laver still being a solid player at age 31 isn’t super alarming. In fact, his compatriots Rosewall and Newcombe also had impressive longevity if we view it through a modern lens. And Bill Tilden was still playing a few tournaments during the early 1950’s when he was almost 60.
As tennis got more physical in the 70’s, player careers shortened, so you had a lot of ATGs who were already starting to decline by 26 and were considered ancient by 30, like McEnroe, Borg, Becker, Lendl, etc. Another major factor was the succession of major technological changes within the sport during this time, like the switch away from wood rackets. Player styles became obsolete earlier into their careers than before.
We haven’t had any such changes since the advent of poly in the late 90’s and early 00’s which helped send the S&V players packing. And with the advances in nutrition and training like you mentioned, player careers have been prolonged.
However, I don’t think this has resulted in players hitting their primes in their late 20’s and early 30’s as some have suggested. Most players still get into that peak level in their early to mid 20’s, IMO. I think its effect has only been to slow player decline. What used to be a quick fall over the cliff has now turned into a glacial slowing. For example, Fed was in a state of roughly gradual decline after 2006 (although I think he can absolutely still be considered prime in 2007-early 2010), until injuries finally forced him out in 2020/21. In the 90’s that might have happened ten years earlier in his career.