I've seen a few folks looking around for something like this. I have an age limit of 16 on our forums. While this isn't an actually file you can click and download and here you go. It's very simple to do, and will take approx 10 minutes.

In your forums location /core/vb/library there will be a file user.php
Do a search for 'needsCoppa'. You'll find a public function near line 753.

Look for:

$cutoff = strtotime(date("Y-m-d", time()) . '- 13 years');

And change the 13 to your age limit.
also 2 lines down from that change

$cutoff = strtotime(date("Y-m-d", vB::getRequest()->getTimeNow()) . '- 13 years');

Again. Change 13 to your age limit.

Then go to your admincp. Language and phrases, then Search in Phrases. Make sure to select in Phrase text, and variables. Search for coppa.
Edit all the text that has 13 to the age limit you desire.

Then in admincp, under settings->options then select User registration options. Look for Coppa registration System. Change this to deny access for people under age X.
I recommend changing COPPA Registration System Cookie Check to yes as well.
Save the changes.

That's it. Your done. You now have age restriction on your site.