I'm all about rapid advancement, myself. I hand out roughly 1 level per game session, or maybe every other. I reward them as the story dictates: complete a major arc of the plot, gain a level. I do it because the players like their powers, and they like to use them, and to see a growth, and to give a sense of increasing danger as the PC's get closer and closer to the final showdown.
There really is no need in my mind to rationalize the speed other than the fact that the PC's are the main characters, the heroes, a special cut above 95% of the other population (the remaining 5% includes other adventurers and their enemies.
). Sure, your average 12th level commoner is going to be an aged old codger, but your 12th level world-saving PC is seeking out challenges on a daily basis that the commoner doesn't face. If that Com12 became a PC, you bet he'd be gaining levels more quickly than staying at home on the farm.
At the same time, you bet that necromancer they faced off against at 5th level has gained his own experience in surviving an encounter with the PC's, and isn't going to be the same when you face him again, even if it's just a week later.
I've had fourteen year old princesses wielding
wish. True, she was mastering a spell the local sage has spent his life attempting to concieve, but in the D&D world, if you sit in a tower and read tomes, you don't get any better. Think of it as excersize, only instead of a six-pack, you gain more
magic missiles.
And yeah, the fourteen year old sorcerer-princess went down in history, as every PC should have the potential for, IMHO. She wasn't normal. She was a normal PC, yeah, but PC's aren't normal in the game world. Even if they start off as the son of a blacksmith, they end up dragon-slaying masters of epic horseshoe-forging. Or they die horribly trying.