When I say -that-, I mean the potential of learning. It is never lost. We may forget (or not be able to consciously remember, yes this is different from forgetting), but we can always relearn.
Why we're smarter before (usually I guess this pertains to those older than "25" or so?) is because we exercised our brains more, I believe. Check on elementary and the like. What did you do for most of your day?
... nah, T. It actually does become harder to learn as you get older, for the vast majority of the population. Vaguely remember something about pathway formation (?), though I've badly forgotten the details. It's a very well known neurological thing, last I checked the research on it (though, being fair, this
was several years back. Maybe they've found out new stuff while I wasn't paying much attention.). Not just folk psych, actual neurology studies.
S'one of the reasons it's a
lot easier to learn new languages when you're young than when you're old (and why mono-lingual education systems are honestly practically criminal, from an educational efficiency standpoint. Languages are best learned young.*). Your brain's literally better wired for it at the time.
Doesn't mean it becomes impossible to learn (for most -- there
are degenerative or brain-damage conditions that
do make it impossible), but... not
as easy. Will's part of it -- you won't learn if you don't want to -- but th'brain's just not as good at learning once you're past full physical maturity.
That said, staying as mentally active as possible does slow down the change (and there's still plenty of older folks that can pick up new tricks and whatnot just fine. Harder doesn't mean impossible, it just means harder.),
and make it less likely for varying degeneration scenarios to show up.
* Also means that, yes, how a lot of stuff (Like, say, math or coding) is taught in most educational facilities are kinda' backassward. Languages -- including math, logic, basic programming principles, and spoken/written stuff -- are best taught young, and through
use. But we don't, in general *vague mutterances*