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.
They actually do this in sports games, sometimes. Fight Night Round 3 come to mind. If you create a boxer, his stats are low but when he trains he makes leaps in his skills because he's young and limber. As you fight toward the championship (there's usually about 15-20 weeks between fights) your stats get higher but the gains from training diminish, to the point where at the end of the game your stats are pretty much set in stone, as they gradually diminish and all you have to train constantly to keep them static.
As for D&D, age comes into play with more than just epic level spells, it's just that everybody ignores them. Haste, for example, ages characters affected 1 year every time it's cast. I can't remember if it's 2nd or 3.5 or both, but if BG2 played by those rules, the party would be octogenarians by the time they made it to the city gates.