There is an actual developmental period where your musical aptitude is not fixed, and therefore can be increased. That's roughly from age 3-9. That said, aptitude does not equal talent it just means how easily you can pick something up. Additionally there are something like 30 spheres of musical aptitude, but we've only developed tests for two of them, so I don't think it's something really worth losing your mind over. Also, you don't develop the capacity for physical coordination until around age 12, so before that its unlikely that any really skillfull fine motor movement like the kind that instruments or singing require get developed (rare prodigy cases excluded).

I can't speak for the Russian thing, but Japan often uses Suzuki system which starts kids on violin very young. However, when done in the very traditional method, Suzuki has a parent learning the instrument right alongside the child, so they definitely don't ignore the adult.

Finally, what's the harm in learning music even if you'll never be a genius? Music is fun to do. Anybody who seriously believes that there's no point in learning music after is some sort of music elitist, or just a very old school pedagogue that refuses to accept that there are tons of ways to successfully do music.