first, to join you need to be 13, but this is the internet, and honestly, whose gonna enforce that rule?
since you're jewish, i'm guessing your bat mitzvah is coming up, or happened. i think that's probably the time you should tell them since it's a coming of age thing and you're agreeing to go forth in your faith. there's something similar in wicca for kids, but they're asked right there if they want to continue on the path, if they say no, it's no big deal. i wasn't jewish, and i'm not friends with a lot of jews, so i don't know the ceremony that well, but that would be the best time, before you go through with it.
if you don't feel comfortable, leave it, but you should tell them sooner rather then later. your parents should understand and support you, but if you think they won't leave it, just remember some day you need to tell them. meditate on it, the answer should come to you.