As a player, the highest I've ever been is 13th, in a 1E game that started at 1st.
As a DM, however, I'm currently running two Epic games- one around 23rd level and the other around 36th. The 36th-level game started at 1st level 6 1/2 years ago (well, almost- April of 2001 had the first sessions) and still has one character from the very first party of four in it; two PCs who joined the game at about 3rd level are also still with the party. Others have come and gone over the years, but only one member of the currently-seven-strong party started out in double digits. The rest have been going for years. The party took about 4 1/2 years to reach 20th, playing from 1st; checking the logs I see they hit 20th during the Bastion of Broken Souls run in December of 2005. The first members went to 21 at the end of that adventure.
I must admit to being very curious what 4th Edition is going to bring, particularly since my main campaign is already beyond its currently-stated limit of 30th level.