The joke's funny because it uses video game lingo to talk about aging. Instead of just saying you're a year older, it says you've 'leveled up.' It makes getting older sound like something exciting and game-like, which is a clever twist. And the way Homer is delivering the news, like it's something urgent or shocking, adds to the humor. It's a fun way to make light of the fact that we're all getting older, and sometimes it feels like it's sneaking up on us.
If you have some gaming friends that have a birthday today, this will be perfect for them.