The first I met when I was in my mid-20s and he was about 17. Gifted kid, absolutely. He'd published a programming book (!) and could do things with JavaScript that, in 2001, nobody had even thought of doing until the mid 2010s. Totally undisciplined of course -- we hired him and he refused to follow any instructions, got bored easily, thought he knew better than his engineering manager, and was ultimately fired after about 3 months on the job. Haven't kept up with him but I imagine he's gone on to do amazing things.
The second was someone I had the pleasure of working with over the past few years. He's early 50s now and far and away one of the most productive and intelligent engineers I've ever seen. Thinks about problems deeply and thoroughly, codes quickly, optimizes when and only when it's time, and is the purest measure of productivity I know. All the lessons one learns over a career in engineering put to use. Proof that hands-down, the ageism in SV is totally misplaced when it comes to skill level -- I'd work with that 50-something over any 20-something "rock star" any day of the week.