Too many times humans apply human behavior to animals.
You have to remember that chickens have absolutely no concept of the term rape. He sees a female, no matter what the age, and instincts along with hormones kicks in. I have 7 pullets that are 18 weeks old and 6 that are about 12 weeks. When I got the younger hens, they were about half that age. Our roosters didn't start to pursue the younger hens until they completely feathered out and their pursuit of the older hens is down right clumsey to say the least.
They do not know any better. The hens are maturing slower than the roosters, the roosters want to breed, and the end result is awkward as the roosters have not yet learned how to properly court the hens.
In the mean time it is hard sometimes to be a silent observer to chicken behavior, but what we call rape, they call normal chicken love.
If you are worried that the roo is going to injure the younger birds, then perhaps removing the younger girls till they have a chance to mature more is in order.
But please remember that your Roo is just doing what comes naturally.