patz I know 12 seems young to be 'curious' but IME it's not - we found evidence in search histories on our PC when DS was that age (and like you, we then finally got around to putting security on the PC!) - and speaking to other mums at school, etc, it seems that's around the age the 'curiosity' starts.
I was really shocked - DH took it much better than me and had a quiet 'chat' with DS about it. Left up to me I would have probably freaked out at him at the time, which I realised afterwards would have been a big over-reaction.
DH's take on it was - at that age he was 'curious' too, but back then you had to sneak a look at your friend's dad's porno mags behind the bike sheds, or make do with page 3. If computers had been around then, they would have been the obvious place for 'curious' and computer-savvy boys to look, just as they are now.
One tip (tho not possible if you have laptops, I suppose..) is that we now have our home PC set up in the dining room, in full view of everyone. The children don't have pc's in their bedrooms, so there's no 'sneaky surfing' anyway. However if you sort out the security side, I suppose you're pretty safe anyway.