I vaguely remember an MMO that actually did that, or intended to do that and never made it to market. (I'm a little fuzzy on which.)
Not an MMO, it was a standalone: Fable.
Fable wouldn't kill your character when they got too old. In fact, while it's been a few years, I'm pretty sure Fable simply used your character's age to indicate their level, with a few scripted exceptions that would age your character (like the prison).