I think it got old when they came out with Halo 2...
Seriously, it is the same game, just with a few different weapons and storyline. The storyline isn't what most people play that long anyways, it's just the online.
I say its getting old...I own Halo and Halo 2 and after endless playing on halo 2 multiplayer I got bored of it and when Halo 3 came out I didnt bother even looking at the cover...I just new its another FPS with kids going "sword is the ultimate weapon!", "You cheater!", "I will F***ing pwn you! *he/she get killed second after saying that*" and the usual Red vs. Blue lyrics.....
By the time they finally put out Halo 3, it was very old. The campaign was worth some co-op time, but the multiplayer was more of the same and I was tired of it before the beta ended.
Halo 2 multiplayer was more enjoyable, but mostly just because the maps were better, and yet I wouldn't touch any of the games now unless it were for co-op play. I'd say that means it's old to me.