I know Vbulletin had the COPPA (The Children's Online Privacy Protection Act) check that prevented registration of users under the age of 13. This is a legal requirement which is bound to become more stringent, (See current California legislation regarding minors online privacy) As a step forward from the simple turning away people at age 13. Make it a variable, so that an admin can set the minimum age.
To change that settings, click on options (under the home tab)
Navigate to Users and Registration, and activate the checkbox (on by default) next to "Require date of birth and minimum age:". You can specify the age in the box below (set at 13 by default).
There's no built-in COPPA form, but it's really easy to make your own and convert it to pdf. You can even make a quick link to it by editing the "sorry_you_too_young_to_create_an_account" phrase under the "Appearance" tab, so when someone is denied registration on account of their age they can download the form. IMO, this is the better route; I never liked the default COPPA form in VB...WAY too messy, and having your own you can specify features particular to your site (such as a chatroom) to be included.
Guess the only downside is you, as the administrator, would need to manually create their account when you receive the COPPA form rather than clicking a box.