In this age where almost everything is in a central database (I will share my thoughts on that elsewhere), probably the best way to do a proof of age check is to have the person type in their driver's license number and state/province if they reside in the USA and Canada. I know a few other sites that do this to reduce traffic from underage people.
I would recommend having the age verification done on a secure server to prevent identity theft as well. We cannot stop who reads the forums, but we will hold fast to our standards on no underage persons registering here.
To our friends who are from other countries, I know the laws are probably different where you live, but this site is maintained in the USA; USA laws apply here.
Thanks, John