I'm in my 30's, so I can't say there's any game that I've NEVER gotten tired of, (except Age of Booty, but I've only had that maybe 9 months). However, I can list some games I played much longer than usual.
Age of Booty (XBoxLiveArcade) a simple pirate RTS. I still play after 9 months. I've only ever enjoyed 2 RTS games. They're both in my list here.
Star Trek: Armada (PC) The only other RTS I've ever liked. I couldn't get into its sequel at all though.
Black (Xbox) -especially fun after unlimited ammo is unlocked. Many reviewers called this game "gun porn". It's the best, most visceral "shoot everything that moves & watch awesome explosions" game I've ever played. Excellent sound too.
Soul Calibur (Dreamcast) Really, who doesn't know about this game? I was surprised a fighting game could have such an interesting single player campaign/quest mode.
Iron Soldier 1 & 2 (on Atari Jaguar & Jag CD) Excellent mech game that's not overly complicated. Plus I didn't even have a PC back then. At the time, it also had the longest draw distance by far I'd ever seen in a game as well as smooth frame rate & great graphics.
Eternal Champions on Sega CD -(which is considerably better than the cartridge version) Superb production quality, tons of characters, tons of moves, tons of fatalities, great sense of humor mixed in with a great sense of purpose as to why everyone's fighting.