While I don't mind characters shooting up in levels fairly quickly, I do have a problem with them going from, say, level 1 to 20 in the span of a year. Generally speaking. If they hit it within the span of 5-10 years, though - no problem. That span of time will probably entail more trouble for them then 50 years for most people.
I really don't see any problem in slowing down their advancement, either; from levels 1-8, travel alone tends to add a lot of downtime. I think my Scarred Lands game has had a span of about 4-5 game years, during which the PC's, who started out at level 2, have only just hit level 11 or 12.
If travel time isn't a problem, then there's still downtime between adventures - I find it highly unlikely that, the day after the ogre band that was terrorizing the village is taken care of, a necromancer comes to plague the valley. So after whatever adventure I've had planned goes down, the next one won't likely come about for weeks, months or possibly even years.
Even little things like scheduling meetings with NPC's in two days hence helps to slow down level advancement to a degree.
As for the human/elf thing, the typical human in my games tends to be around level 1-4, whereas most elves would be around 4-10. Beyond that, PC's are the exception, anyway. NPC's don't accumulate XP like the PC's do; if I have a recurring NPC, it will level up at about 1 level for every 3 the PC's gain (the antagonists, anyway, usually start out at a high enough level that this is more then enough for whenever I intend them to finally face off for the last time). Everyone else is pretty much going to stay where they are and even some of the recurring ones. The hypothetical town priest where the PC's live and make their base would likely never, ever advance in my games.
In regards to exceptional individuals like Joan of Arc or Alexander the Great...I really think they just show some of the difficulties with the level based system, where individuals uniformly advance, rather then having it broken up. Personal power in no way reflects influence over others. It seems more likely those individual's possessed a number of feats such as Skill Focus (Diplomacy), Leadership and particularly high mental/social ability scores (Charisma most importantly). Heck, I've always found the idea of the leader in a group being the highest level to be a bit silly at times...only in the most undeveloped of societies is the strongest likely to wind up leading. The brightest professor isn't likely the dean and the most deadly warrior probably isn't the general. I'd also peg most of the worlds exceptional people as being somewhere around 8-15, anyway, if I were to give them levels and just focus their abilities to near ridiculous degrees in what they should have (like with Skill Focus and the double bonus skill feats).
So their youthful talent isn't a matter of level, it's a matter of not being glutted with things like Power Attack...
But, I digress. There's my general thoughts on the matter.