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.