Given it's always taken me forever to get around to finishing the main quest, most of my character haven't (and probably never will). Those that have where my 1st three characters (Imperial, Breton, Wood Elf) and my 5th character (Breton battlemage--Saozig a' Dhubh in my sig). Don't recall at what level but given how easily distracted I am with finishing questlines, I doubt any were below lv 50.
I know when my last one did she was in the early 60's level-wise, was already Arch-Mage of the College and had hit 100 in at least 4 skills. She had all Daedric armor and weapons, double-enchanted everything and when she defeated Alduin she basically Dragonrend'd him 2 or 3 times and while he was grounded pummeled him a few times with her Daedric warhammer that has an Absorb 27 Health/Absorb 27 Stamina enchantment. I had it on Expert difficulty because I was getting annoyed with the all the Draugr Deathlords in that area you have to go through to get to the dragon priest and the portal to Sovngarde, and bumped it down from Master to Expert so she could run through there without being Fus Ro Dah'd by five Deathlords at once. But I forgot to put it back on Master for the final fight, so it actually wasn't that hard for her with Dragonrend and that monster enchanted warhammer of hers.