My 2 c.p. on some issues:
It's D&D. You start at 1. Always.
I will go with Gygax (in the 1st DMG) on this, as concerns old D&D. If you've
never worked your way up from 1st level, then you should be allowed (indeed, encouraged) to have that experience of wonder and discovery. It is also good to learn the skills of survival with easily and quickly replaced characters, rather than to go through high-level ones at the rate so easily attained by inexperienced players.
(To the very extent that it takes longer for error to get them killed, learning is delayed while attachment to the persona is increased. The consequences tend, I think, to be unpleasant. At least, I have not seen the same
sturm und drang among players who started with the rapid turnover of 1st-level characters.)
Experienced players, however, already know the ropes -- and the usual monsters and spells and magic items like the backs of their hands. The thrills that come with one's first forays into the 'world' of D&D have a lot to do with mystery, with not knowing just what is going on, how this works or what that does.
To maintain the challenge and excitement of the game, it makes sense to me to give old hands higher-level characters if they do not already possess such (that one is willing to introduce to one's own campaign). My own custom is that this should be a one-time, "new to the campaign" courtesy. After that, it is up to the player to cultivate henchmen as "back-up" characters -- or, failing in that, to pay the price by starting PCs at 1st level.
(The latter is not so harsh when it typically means, in the long run, ending up just a level behind one's former peers. In 3e, I think it would probably make a more significant difference.)
Some people just have no interest in starting at less than, say, 4th. That happens to be about where I think I really hit my stride as a DM, too. There's a much wider variety of stuff one can throw at heroes, they can handle more in rapid succession, and most of the time it's literally impossible for one hit from a kobold to lay them low.
Now, there is the complication that people can be very well acquainted indeed with D&D 'tropes' without ever having encountered old (or even any) D&D. The original game has been a seminal influence on computer games, genre fantasy fiction, and other media. Those who have played a newer D&D are especially likely to encounter misleading similarities and confounding differences.
Moreover, it is an important question whether, probably steeped in quite different expectations, they are even interested in playing such relatively weak and fragile characters as 1st-level ones in old D&D. At the very least, it may be meet to give them maximum hit points (as does 3e).
In these cases, there is a lot to learn in terms of the skills to survive and succeed. However, the trappings might not be as novel and engaging as they were to those of us who first encountered D&D in a world not already so much
a product of the game.
I'm going to say 10th to 14th, regardless of edition -- before D&D gets stupid hard to run.
In my experience, players have nearly always retired characters from conventional dungeon adventures shortly after attaining "name" level. The higher levels, including the spell levels introduced in Supplement I (cleric 6-7, m-u 7-9), seem to me mainly of utility for deploying opponents strong enough singly to pose a challenge to a party of high-level PCs (which can remain quite formidable despite much attrition of resources). The high teens are literally "godlike", with reference to the books detailing deities!
The game increasingly comes, in many eyes, to be reminiscent more of Marvel Comics super-heroes than of "sword and sorcery" fiction or even mythology.
