I say 16 should be the age because by the time we are 16, we know a lot about our sexual preferences. 16 is the legal age of consent in most places. So, I don't see why you can consent, but you can't buy or watch adult material. It doesn't make sense.
I have to agree with others. As if having to click "yes, I am 18" is going to make a difference. All you have to do is type in the word Pink in google. Even with the "safe" search on, you can see girls in their panties, bras, and topless. So, it's not hard to find nudity that would classify as porn.