Compare with other countries where alcohol usage is not taboo, where the drinking age is low (and/or unenforced), where teen alcohol abuse is a far, far lesser problem.
It turns out that making something taboo and setting up some rudimentary roadblocks to its acquisition does not have the effect on consumption that you think it does...
Speaking anecdotally, growing up in Canada there was no one under the drinking age who couldn't get alcohol if they wanted it. Hell, half of us didn't even like drinking, and we did it for the taboo-ness of it all.
If porn was found to be - without a doubt - bad for a certain age group, then yes I would get behind something where the government would limit its access to minors. This law, however, further stigmatizes porn instead of embracing it as just another weird thing humans do for entertainment.
(I want to make it clear that I'm not referring to illegal pornography. "Porn" where people get raped or that deal with minors is illegal)
...why do you think this? If you raise a child to be intelligent and independent, education is pretty much the only effective way to keep them away from something.