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..
Funny thing is, I'm a "Google Trusted Tester" yet they never give me anything to test :P
@ChristopherBurton: They are getting the date of birth from the information you entered when you created your Google Account. I don't think you can change it.
https://plus.google.com/i/LjJGdQK3S6E:zBtJgvwGfzY
Glad you guys are finding it useful. Don't be afraid to invite others with the link.
@ChristopherBurton: They are getting the date of birth from the information you entered when you created your Google Account. I don't think you can change it.