The fact that most engineers are young is much more to do with the numbers in the profession doubling every five years in the last twenty years imho. If you were an engineer twenty years ago, there are now sixteen times more engineers. Your peergroup is going to look pretty small therefore. All of these younger engineers are going to get older too of course, and the demand won't keep on growing at this rate, so inevitably the ratio of young vs old devs will even out.

Sure, some devs become managers but there are only so many managerial jobs. Some also drop out but not in vast numbers. From my own experience, I see quite a few of devs my age (40) and older. For the most part we're senior engineers, some are managers. I don't know anyone who's left the profession. Just my anecdotal evidence but I do feel this phenomenon is overstated.