It is utterly retarded though. Last night I was checking out Alpha Protocol on Steam, and it asked me to confirm my birthday. Being sort of on auto-pilot (it was 5 am...), I changed the month to November (which is when my birthday is) instead of the year, which meant I entered November 1 2009 as my date of birth. Steam then kicked up a great big fuss telling me in all caps "Sorry, but you're not permitted to view these materials at this time", and any subsequent attempts to enter the info again was impossible until I restarted the client.
It might be a law or regulation, but it's an extremely stupid one that isn't going to "protect" any minor from seeing the product page and only serve to annoy the 95%+ of users who are old enough to "legally" access it,