I've only watched the show "Seinfeld" a handful of times; and, from what I've seen, I think if there were to ever a be video game made from it, it could more than likely sneak by with a "T" rating. Perhaps the age of 13 or up could be okay to play.
Seinfeld was always very good about handling topics that could be considered mature, such as sex, in a way that both entertaining and safe to watch as a family. If Seinfeld was made into a video game, any attempt to make it an M-rated game would be totally unfaithful to the original series and would probably lose all the charm that made Seinfeld such a hit.