I was talking about this with a friend just this past weekend. You get about 10-12 years of the really good stuff -- where they're cute as heck, love you unconditionally and think you're the greatest parents anyone could want. Then it's gone. I guess grandkids are gonna be a close second, but I don't know that from experience.
Don't get me wrong, life's pretty good, but I'll never see those days again. It is pretty satisfying to see -- when things start to click and your kids start to show that the things you tried to teach really did stick. I'm just now getting to that point with mine, even #2 son.