Dale Jr. claims he'll retire at 40, so you won't have to watch him in your waning years...
I'm thinking that might be the trend, though. Make a bunch of money, win a few races, and get out while you've still got all your faculties. It's unfortunate, because I rather prefer the older guns to the younger guns, with one or two exceptions. I'd hate to think that racing will become exclusively a young person's sport, like all the others.