Since I’m already using some other Google services (Gmail, iGoogle, Google analytics etc.), I didn’t have to pick a date of birth at all. It just immediately filled in my normal Google account details, so maybe logging out first is an option? I think you have to be 18, but I think that restriction will be removed once the limited trial is over.
Apparently, you can’t edit it, it seems they disabled that function. Just look up “google profile edit date of birth” and you will get lots of results of people saying they can’t edit it. Interesting..