It seems a lot of parents are very upset about their children under age 13 having their Google accounts deleted. They seem to think that since Google didn't explicitly ask for an age at the time the account was created, that their young children could have one. It wasn't until Google Plus was launched that they learned differently.
Children are having their Google accounts deleted, Picasa photos, emails to Grandma, school work in Google Docs, and all, for violating the age restriction, when they enter their age when joining the Google Plus social networking service.
The Google Terms of Service clearly states in Section 2.3
2.3 You may not use the Services and may not accept the Terms if (a) you are not of legal age to form a binding contract with Google, or (b) you are a person barred from receiving the Services under the laws of the United States or other countries including the country in which you are resident or from which you use the Services.
While most websites have a clause in their contract barring use by those under 13 years of age (there is a law restricting that), Google has chosen to word their Terms of Service in such a way that it bars any US resident under the age of 18, because minors under 18 can not legally form a binding contract with anyone.
Let this be a warning not only for those under age 13, but for those age 13-17, as well. Google may delete your account and everything in it, if they find out that you lied about your age by agreeing to their Terms of Service. If you are under 18 and have a Google account, yes, you did lie to get it.
Always read the Terms of Service. If you don't, it could come back to bite you later.