You need to do things to gain experience. Doing time is not enough. A 20 year old can in many cases have done and experienced much more already in life than some 40 year olds. The large majority of today's mass population have identical lives that are staked out for them from the cradle to the grave, and it is true that most only gain experience by accidents which accumulate over the years. But many people live real lives already from a young age, and have the experience which that gives.

The implication goes the other way: someone who has done a lot of things and learned many lessons has lived a longer time. Not that people who have lived longer necessarily drew valuable lessons.

It's less likely that a 20 year old has done as many things, faced as many challenges, and drawn as many lessons, as someone who doubles them in age. They simply haven't lived long enough to be able to tell if their lessons validate or generalize well.

Even the perspective of someone who has most of their life ahead vs someone who has a few decades more is wildly different. Someone with most of their lives ahead can course-correct more easily. Someone who hasn't had their mid-life crisis yet, etc, etc -- you get the idea.