An interesting way of constructing "heroic" campaigns, but I do partially agree with Whizbang.
Interestingly, as I like to run campaigns where the PCs aren't automatically special, but have to prove them selves so over the course of a campaign, I take a very different approach to demographics.
Basically, if a young punk PC challenges a couple of semi-grizzled old workmen in a bar after his first adventure, he can expect to get his ass handed too him.
All first level characters are basically teenagers.
With this in mind, I have all humanoids start of at a classes starting age, and then accumulate 1 level equivalent of xp in a class for every 5 years of normal practice of that class, up to 5th level. Then 6th -10th level take 10 years each, 11th to 15th 20 years each, 16th-20th 40 years, etc
With the caveat that you can't normally advance to a level of a spell casting class that would grant access to spells you can't normally cast, and special training is required for PC classes (usually from childhood) excluding favoured classes (except humans, who don't have one). Multiclassing is very common
Thus, the militia (most adult males) in an average human frontier village will be something like commoner 3/warrior 1, and a venerable elder might be commoner 3/warrior 2/expert 3/adept 2. In an elven settlement on the other hand, an adult would be something like: warrior 2/wizard 5/expert 2, and an venerable elder, warrior 2/wizard 10/expert 8.
The power of the nobility would be based on the fact that they intensively educate their children in a PC class from a very young age (squires etc), so they start off better, as rather than a human starting their progression at 15 they do so at 10, and live lives much richer in experience. Thus, a 25 year old human noble may well be a war blade 4 (Note that war blades replace Fighters IMC) if the eldest son, or a wizard 4 if the second son, or a cloistered cleric 4 if a daughter.
Of course, exceptions and prodigies still exist, such as the PCs, but also other high level characters from short-lived races., but most high level beings come from races that can expect to last a long time.
This of course, has effects on the nature of the campaign world. The survival of the long lived rates despite their incredibly slow reproduction is explained, as their sheer individual might means that attrition isn't such a threat. Although their are lots of high level characters around, most are so multi-classed as to not over-shadow high level PCs, whilst keeping the low-level ones careful. Low level adventures still exist, but they are very small scale affairs, and generally don't involve huge amounts of physical conflict. You're not saving the village at first level, you're scouting out the tracks of something mildly unusual the local ranger has seen that isn't interesting enough to follow himself, or you're finding the source of an infestation of (normal) insects - essentially a 15 year old kid and that is the type of work you'll be doing.
If you ant to start of doing something more interesting, w begin at 3rd level.
