I've asked this question dozens of times over the past year and a half.
What sense does an age limit on a web site that was, for the most part, an adult site that also had 'work friendly' content?
The solution to enforcement of an age minimum is fairly easy but would kill much of the traffic over at 4chan.
They could have done what Lotax did and make 4chan a members only site.
As it stands now, no one can ever justify having all age content on 4chan as long as /h/ /b/, /c/, /k/, /pol/ and a half dozen other boards that cater to extremists who do nothing but shit post adult content exist.
The underage rule is total bullshit in the face of what you can still find on 4chan.