I'm 35 so I state my age honestly, heh.
Topless ladies can be viewed by anyone of any age. In fact the very first thing most people see when they come out of the womb is a topless lady. So anyone blocking access to such a thing due to age is a hypocrite.
I think that generally when it comes to this kind of stuff, most "you must be over 18 to view this" disclaimers that I've seen have content behind them that is incredibly tame when it comes to anything sexual. I don't see what so diturbing about exposed genitals for example when the entire human population has genitals that they can look at anytime they want. On the other hand, while sites like YouTube will censor anything that even looks like an exposed nipple, they freely show snuff films, and footage of people being shot. I've seen Army sniper footage of people's brains exploding etc on so called family friendly sites... I can't believe it didn't have a warning on it.