Children's access to adult sites

Based on “CAPTCHA. Top 10 worst. ”By langraf

In this topic was posted one interesting captcha with the equation.

With one of the habrauys, I began to discuss the applicability of such a captcha. My opinion tended to the fact that this captcha will cause rejection from the site and it will lose visitors.

BUT! .. I thought to use this captcha for ... adult sites. I think many of you have children on this site, and some of them thought about restricting their children’s access to adult sites.
')
So, maybe the use of such a captcha at registration is the way out? Those. a child entering such a site will have to register and solve an equation or answer a question that any student who is 18 or older should know in theory. Someone will say, but you can find the answer to the equation or question on the network, but then the child will make an effort and this will again have a positive effect. These sites will fill the gaps in Russian education by motivating these children to access adult sites :)

The theory of raw ... but even so.

What do you say?
I wonder your opinion.