If you use people you are reasonably expected to kill and their level as an indicator, lvl 30 (Marara and the vampire leaders) seems about it. A Morag Tong quest requires you to kill a lvl 25 character, and the main quest targets a lvl 26 character.
Looking for high level characters, apparently there are some non-aggressive npcs in the Dunmer strongholds (Missun atop Falasmaryon, Alfhedil in Falensarano and Abelle in Valenvaryon, all being master trainers).
I don't think so, afair higher levels are gained more slowly.