Appearently they live on a planet with 4000 some days in a year because if you only count the time each show/movie lasts for plus the inferred time (when it says something like "two days later") you come out with at least a year of content, even then, correct me if I'm wrong, but I believe he experienced several of his birthdays in the first six seasons, but he's still like eleven or something like that. Conclusion: don't ask questions, just follow blindly or you'll break it