It depends on your self control. If you are an idiot, you can get yourself in trouble even on a scooter or 125cc bike. I've got my ZX6r 636 04 right after I've go my license (at 28). The interesting thing about sportbikes is that they make most of their power above 7000rpm so they aren't that scary to start on. You can grow with this bike and it has a lot better brakes and handling than the smaller and cheaper bikes. As for the twins. It can be difficult at times, as more often than not, they have more torque than a sportbike and they make more of their power sooner (at least it feels that way). So to sum it up, with good self control you can start on 600cc sportbike with no issues.