It’s true to some extent. Most white belts over 30 end up injured. I was a competitive ultra runner for years so the grind made sense to me and although I am not explosive, I could already wear most white belts down after a few months of training. Mostly my point was around skill development and it is a relative thing. Still, athleticism is y-intercept. Slope (years of training) can make up for a lot but there is obviously some inherent levels you will just never overcome as an older hobbyist, but you can still get really good. Maybe never elite competitive, but that is a small fraction of young people anyway.
There are a lot of people who go on training jiu jitsu (or other combat sports) for many years and they remain as uncompetitive as they were in the beginning.
Some because of athletic limitations, some because they are unable to put into action what they have in mind, and some because they lack sufficient intelligence.
"Study or practice for enough years and you will get good enough/competent" at the population level sounds good, but it is simply not true.
