This is the only good idea for it I've heard. The age of the character alters how fast XP is gained. However, I see this idea ultimately screwing over casual players, because they may not use their time as efficiently. Which would be at odds with one of Hurlshots originally supposed benefits of helping casuals. I don't see real benefit to it or how it would improve anything.
Wisdom and intelligence are the benefit of experience, not physical age. If a game already has character growth mechanics based on experience, having an artificial wisdom and intelligence boost with age is miraculously both redundant and nonsensical.
Since I should probably be more constructive directly to the musings, here's how I think an MMO game featuring aging could reasonably be implemented. I've established that I don't feel it's all that reasonable to have it in a way similar to typical MMORPG. So, how about an MMOStrategy game? Family lines help to establish armies. In the "peak years" the character fights on the battle lines. In the aged years, he moves to a rear line position and uses the accumulated experience gained from combat to improve overall strategy and tactics. He has children which he can pass on some of this information to, not only increasing the size of the army, but increasing the overall competence and ability of the army with time.
Of course, I'm not sure how well an MMO strategy game would work out from a functional perspective.
Perhaps even this structure could be used for a ground up MMO. A single starting character, opening up new character slots after every few "years" and allowing the aged character to retire and simply provide "family benefits" which strengthen the newer characters on the account. It would have to be sandbox styled and designed with a greater interest in larger goals than character development and investment.