The best programmers I've hired where those who love programming. It's probably the case for any skills. The critical question I ask is if the developer has personal programming projects. It's usually an indication that the person breathes/loves the stuff.

Starting at an early age is great but doesn't mean much. Continuing doing it with dedication and just for fun into adult life is awesome. If you're still sticking with it professionally and still liking it, chances are you're a great developer. But of course, that kind of path works from whatever age you start getting into it.