Just Because I wrote it....
The mechanics of leveling are absract and I treat them as usch in my game except for the fact that it does take time.....to get better.
I love this question it is one that has fascinated me as a player for years and now as DM I get to act on the things I found silly and well, silly.
I recently played in a great game with a fantastic story line and plot twists; however, my enjoyment of the game was cheapened on two accounts.
1. We saved the world, basically as about 12th level characters, I wondered where all of the epic guys and truly "world influencing" (15-20th level) guys had gone? Perhaps they were busy with other more important quests etc. At this mid-level range I think that saving a town, defeating a local warlord who is part of a worl domination plan or leading an small army are appropriate, not world saving. 12th level guys would be short lived in this situation, I think of all the assassins that would be sent out to stop these upstarts as the epic NPCs and such began to fear for their places and roles in the world.
2. The passage of game time.... all told in a little under 4 months of life my 1/2 Orc Fighter.... joined an adventure group took a couple of levels (all of 4 days game time), studied and became a ranger took two levels, studied wizardry too, took 3 levels of that, joined the spell swords, and finished my lustrous career at all of 16 years and 4+ months old, a hero having saved the world from an age of chaos. Whooooho!
I mean it is fantasy but come on, I this type of game when do wizards, research spells and make stuff. (That, was a whole n-other issue with us raking in about 10% of the wealth on the wealth by level chart, I have declined to play with the GM in future scenarios because there is no opportunity to do any of the high level stuff like make magic, stronghold build, etc. but that’s another topic altogether.)
In my own game I am going with a level a year approach, that being the hero's spend about a couple of weeks adventuring per-year the rest is training, research, etc. In an abstract way they don’t have to find instructors, spend cash etc. During that time I give them ample opportunity to earn money and stuff since it does take their time. Now they are at about 10th level and as such they will go to the level every three year schedule, this is because that now they all have minions, strongholds, and other key plot roles to play in the campaign and therefore spend a great deal of time dealing with matters of state, politics and economy. After 20th I'll probably go with a level per 5 plan. All told a human fighter will hit middle age at about 13-15th level and old age around 18th 21st level after that a human PC without magic lifespan enhancement will only live long enough to reach 25th level or so.
This plan allows even the longer-lived classes a chance to get to middle age by the time they are reaching epic status. I don't really think that its fair to make the other races go slower in age progression by levels except with NPC’s who will follow the same schedule for age and levels as humans above, reaching the same points age wise at the same levels i.e. an elf NPC 20th level wizard will be "old" or around 250+ years.
Age for wizards is a boon and I allow my players to start at middle age if the want so they can get the non-physical advantages that a younger fighter gets by virtue of an early start.
Anyway, age is a cool part of RPGs that is often overlooked by DM's who want a faster paced game. I still manage to achieve a fast paced plot wise even with the slow down of game time.