I tend to keep my decks together once I've built them [I think the only EDH deck I've torn apart since I've been serious about the format is my Geist of St Traft deck] so I have a few that are getting up there in years.
Grimgrin, Corpse-Born deck is primarily a mill deck with zombie tribal beatdown and aristocrats strategies as backups. I built this one at some point during Innistrad block, right after someone showed me the EDH format. This is the one deck that I'll spend extra $$ on to foil as it is still my favorite to play.
My Gruul deck is the typical ramp out big beaters ftw. Its still mostly a beast tribal deck because I wanted to run
Contested Cliffs. It started with
Ruric Thar, the Unbowed as the commander [still in the deck btw] but changed to
Xenagos, God of Revels once I was able to get a copy. I've slowly been swapping out the beasts for things that work better with the jolly old satyr. Built this one right along with Grimgrin
Niv-Mizzet, Dracogenius started as wizards and dragon tribal that I made for a friend to play but I've kept even after he stopped playing MtG. I initially tried to keep as many of the older cards that he was familiar with in the deck and no combos, but have since added
Azami, Lady of Scrolls/
Mind Over Matter/
Laboratory Maniac as a win con. Also part of my first wave of EDH deckbuilding.
Olivia Voldaren is vampire tribal with a curse subtheme because I like the idea of curses, not because they are good. Also an Innistrad era build.
Boros aggro that started with
Kalemne, Disciple of Iroas as the commander but shifted to
Aurelia, the Warleader as I got my hands on more of the better
angel cards. Made this one as soon as C15 came out.
Meren of Clan Nel Toth was typical Golgari Goodstuff but I've been expanding on maximizing ETB triggers and stealing opponents' permanents. This one took a bit longer as I couldn't find the deck for sale and had to wait for singles to appear.
I know I've had my
Krenko, Mob Boss and
Sliver Overlord decks for awhile but can't remember exactly when I built them. They are both tribal decks though Krenko has become more combo-centric over time.
So...eight decks that I've had for at least 4 years +, though they may have had some serious changes since first assembled.