Actually everything you've said is a reason why you should join.
Game Jams help developers get a project out the window, even if it's very limited and lacks polish. You might end up with something you build off from and make it a complete game. Plus, you get to say "I've been making games since I was 13 years old", something which others will find pretty impressive once opportunity arises.
If you're worried about your abilities, don't worry, this is just a start. No one's going to discredit you for trying.