> And a growing person whose brain is still developing who continuously memorizes new things will be smarter by the time they're 20 then the same person that didn't,

I dunno, I guess it depends on what we mean by "smart" but I've definitively met (and been friends) with people who weren't able to live on their own by their 20s, although they were very "smart" in school and highly intelligent in general. I've also seen the reverse, dumb people being "better at life in general". I don't think it's as black and white as you're trying to make it out to be.