This is an interesting question. Many websites that have content that would be interesting to a 12 year old will not let a 12 year register with the site. I believe it’s mostly related to the COPPA Act. YouTube simply won’t accept the registration, Yahoo will only accept it if it’s tied to an adult’s account.
The obvious and easy answer is simply to make up a birth date. Beyond facilitating access to the site, this also has privacy implications as using a fake birthday helps protect your privacy. I’ve been locked out of sites in the past when I forgot my password and couldn’t verify my birthday as part of the recovery process because I had no idea what I supplied as my birthday when I signed up. Preteens tend to live in a black and white world, so teaching them that it is OK to lie online may have repercussions that are less than ideal.
The parent I linked to above used his birthday and registered on behalf of his child, which is a good compromise. However, generally speaking, there are a lot of situations online where you are asked for personal information that really isn’t relevant to whatever you are doing. We’ve taught our kids to not provide personal info online and to use aliases in their online games, etc.
Is an alias a lie? Is that bad, or that one of those good white lies? It’s a tricky question.