I couldn't disagree more. I think the game would seem a lot less consistent and have much less story and depth to it if we lost the tales and lessons from the 15- and 20-year characters. Most of the high-level PCs I know are more than willing to use their power and their knowledge to help the younger adventurers learn, if the lower-level PCs want to, and grow as characters. Also live to grow as characters.
If everyone's lives have an expiration date, we lose both our past, in that nobody has firsthand experience of the histories that have brought us where we are, and our future, in that we know we're going to die or be put on a bus (carriage?) once we get to A Certain Age.
I mean sure, we could stand to lose SOME crusty old hoblings*, but I GUESS we can find something to keep them around for.
(*This is a joke, I'm joking at the expense of some crusty old hobling friends who hopefully won't now be stabbing me hello in 36 hours)