Here it's a quick snippet I had to do for this site I'm working on...
I did a quick google search to see if there was a snippet for it already but didn't find anything so I though I would share it here. I though it was going to be a nightmare job but ended up
quite simple:
var legalAge : int = 18; // or 21
var userDOB : Date = new Date(year,month-1,day);
var today : Date = new Date();
var diff : Date = new Date();
diff.setTime( today.getTime() - userDOB.getTime() );
var userAge : int = diff.getFullYear() - 1970;
if (userAge >= legalAge)
trace("lets get pissed!");
else
trace("go away!");