[bryondowd]

18 maybe for physical appearance, but mental maturity and brain development completes in the early-mid 20s. I would definitely be the odd one out at my current age if I were stuck with 17 and 22yo me, unless you were just using physical appearance, in which case yeah, I guess 17 would stand out.

[TheOtherHobbes]

Mental and social maturity continues developing for most of your life. Different life stages have different common problems, and you need to learn different skills to solve them - or sometimes just to cope with them.

In the same way that there isn't much practical (i.e. economic) interest in tailoring schools and college classes to people with different daily cycles, there isn't much interest in teaching people about common problems ahead of time, or running some kind of continuous life challenges training that could potentially do a lot to improve life quality.

In my 50s, one of the common problems my friends have is dealing with parents who are either dying, recently dead, or have a terminal illness like Alzheimer's.

Of course that happens to younger people too, but there are actuarial peaks where it becomes massively more likely that you'll be dealing with a certain set of challenges in a certain decade of life.

There's shockingly little information around about some of these challenges. So it's incorrect to assume that the learning ends after school or college.

You won't learn anything about dealing with these challenges there. You won't even be warned they exist.