It isn't the porn as such.
The problem is the amount of porn that gets into an everyday search; then your into a thread that can be extreme. I'm no prude, and I don't mind at all, but I'd be embarrassed by some of the links that pop up if I was in a public space, living room.
For me; it's not about stopping the kids browsing after a certain age... it's about attempting to block the unwanted effects that they can link to by accident. Lets not forget there are morons out there who find it funny to 'honey pot' folk with fake links. Probably some of the folk in this thread going by some replies!
Yes everything can be bypassed but that's more of a concious decision. Arriving at one of the porn aggregating sites with millions of flash films, by accident, can't be a good thing for a kid that isn't expecting it. Surely nobody is saying a thirteen year old can get a balanced view of life from that?