I voted for 11-15. I like DM'ing at all levels and usually want PC's to start at 1st level, but for me as a DM and the style of play I like, the higher levels allows me to make things the most interesting. I can throw a continuous mix of challenges at them which they can handle without too much downtime.
I do not have to worry too much of throwing too much at them, and for me it is a more fun challenge to try to make a bunch of kobolds a challenging fight for the PC's then to have to figure out which monsters and other encounters are not too much for the PC's to handle.
Part of this is because most of my campaigns are not very magic item heavy (i.e. making REAL magical items (i.e. not potions and scrolls) is simply too costly and arduous for most people to do, so not that many are around), this means that more prowess and abilities will have to come from the PC's themselves.