the highest level characters i've ever had was level 13 in 5e, but the campaign started at level 3 and i joined at level 4, so i guess that doesn't count.
the next highest level character i have is level 9 also in 5e, and normally that wouldn't count since that game also started at level 3, but we level up once every...what, dozen sessions? at most? so maybe that counts.
the only character ive had that started at level 1 was in 4e, and he got to level 6 before one of the players lost their pc and we swapped campaigns.
...no, wait, sorry. he got to 4 and then i retired him for a swordmage to better fit the party dynamic, and the SWORDMAGE got to 6. and that was milestone...so...uh...
i have a pathfinder 2e character at level...8, i think...who got there from level 1 with xp. but that's not dnd. so. hm.