They don't do math anymore?
If she was celebrating 25th birthday for the 25th time that would make her 49 years old, and not 50 as he said in the movie
shareI think it's possible that the writers thought it would seem contrived if they got it absolutely correct, and may take even more people's attention away from the events in the film as they mused about it.
So they were happy to let the character make the mistake.
http://www.youtube.com/watch?v=GAIJ3Rh5Qxs