Same story here. The most talented engineers I know are either in their early 20s or in their late 40s/50s.

Of course it's for different reasons. Something many people don't want to admit is that a lot of talent comes from practice and experience, and someone in their 40s is much more likely to have 20+ years of experience than someone in their 20s.

The early-20s geniuses I know are talented for different reasons. The ones I am thinking about are people who pick up new things absurdly quickly; pick up on patterns very easily; have general experience in enough domains to make quick connections; etc. But some of them are extremely undisciplined (which may or may not be related).

Those same people with age, gain not only experience but also hopefully discipline. A combo like that yields you people like John Carmack and the like: Not just talented, but out-of-this-world "next level".