My current Epic game has been going continuously, more or less weekly (I say "more or less" because a session gets cancelled now and then for minor annoyances like, say, Christmas, Thanksgiving, too many people being unable to make the game, etc.) for slightly less than 6 years. It started way back in April of 2001; I still have log files from those sessions today and could in theory string the whole thing together for one continuous read. It's be a long one- the logs currently take up roughly 50 MB of my hard drive, and that's pure text- no other file types.
Several party members of that game have come and gone over the years, but the core of the group has remained the same since about level 8- and one group member was in the very first session with four people getting together at 1st level to try out this newfangled 3rd Edition of D&D (the psionic elf who specialized in item creation). The next core member (the party tank/melee Greatsword specialist) joined at 3rd level, and the party dragon joined when the PCs were 4th (she came in as a Wyrmling at the time). Next, the party Bard joined around 8th level when the party arrived in an extraplanar city after being rescued from certain death on their first extraplanar trip by another band of adventurers (we ran said other group as a one-shot right before Gen Con of 2002 for reasons I won't go into here- the rescue fo the "main" party was really just a bonus of that game). Finally, the party's sorceress/blaster mage started at 8th, a month after the Bard in real time, as a replacement character for an archer that the player wanted to retire both for roleplaying reasons (he'd just been messily killed by an archmage and resurrected) and because the player himself wanted to try something new.
The party is an average of 31st level now, with several members including the tank and sorceress being 32nd. The tank is a Fighter with most of his levels actually being in a homebrew prestige class geared towards mobile combat and extreme bursts of speed. The elf is a Psion (Shaper) with 3 levels of Sangehirn (WotC 'Psionic healer' prestige class from Mind's Eye articles) and most of
his levels in a homebrew prestige class called Mindsmith which is the item creation specialty I mentioned. The dragon kept going as a pure dragon, since I'm using rules similar to the articles from Dragons #320 and #332 which allow dragon characters to gain age categories by advancing levels in the Dragon class- she's now effectively age category Ancient. The Bard has stayed as a pure Bard all the way up to the stratospheric heights of 31st level (well, 30th with her racial LA), and has quite a lot of spectacular stories to tell these days. Finally, the sorceress went through a period of getting to know her (primitive, tribal) people again after some years of living in exile, and starting at 22nd took levels of Favored Soul of the Nature goddess (by my house rules this means she actually uses the Druid list for spells rather than Cleric), and followed up with Mystic Theurge once she qualified. So she's an extremely powerful spellcaster now, effectively a 28th-level Sorcerer/11th-level Favored Soul with Druid spells.
So to answer the follow-up question- yes, yes I have seen a character go all the way from 1st to 20th.

The elf in my Epic game has actually gone from 1st to 31st.
This is certainly the longest game I've ever personally run, and one of the longest I've ever heard of from others. I still have some old, ridiculously-long-running overarching background plots that can generate adventure ideas for years to come, but in recent days nearly all of the adventure plots have been coming from consequences of actions the PCs took in previous adventures. I intend to keep it going as long as we still have fun, for however long that means.