I have a Tamagotchi V2 and its 8 years (days) old! My V1 only lives about 6 years (days). NOTE: I put in all 5 codes I found on this site, maybe that has something to do with it?
Although it might be possible for a Tamagotchi to reach age 99, I thought that after a certain age it gets so needy that it's almost impossible to take care of it? I know that happens to the older Tamas (e.g. my Osutchi). Check mimitchi.com's Tama logs to see what happens to really old Tamas (they lose happy hearts once a minute etc.)