How old does a rooster have to be before he starts "mounting"???

[Tripp16]

Hi everyone I have a little cockerel thats about 3-4 weeks old and I put him with a young little hen chick thinking that he would be nice since he is a rooster. Well she was very polite but he ignored her for a bit but then kept grabbing the back of her neck and sitting on her!! Is this normal? Should I let him keep doing it or remove him? He is a bit bigger than her....

 

[CherryChick]

I want to say it is pecking order that is going on. But that is just from my experience. When my chicks were about that old I had a really hard time having genders get along. The boys were being very aggressive towards the girls. They didn't try to mount or any thing. But it did cause me to seperate them. They eventually chilled out.
My boys didn't start trying to mount our girls till they were about 13-15 weeks old. So now we have them seperated for a while since we don't want any fertilized eggs at the time. It's up to you if you would like to seperate them. My rooster is very sweet to the girls and they let him do what he wants with no fuss. Even thou he does his dance and pecks me when the girls aren't outside with him.

 

[debid]

At 3-4 weeks of age, it's a pecking order behavior and pecking order fighting is to be expected when you first introduce any two chickens. FWIW, my worst offender for back-jumping was a pullet. She's presently 2nd in the pecking order (to the cockerel).

And as for how old, my cockerel just started his first awkward mounting attempts and he's ~14-15 weeks. I understand that males aren't expected to be successful enough to produce fertile eggs until more like 5-6 months. The mounting behavior closely followed crowing and the development of combs/wattles/reddening of faces on the quicker-developing pullets. I've also noticed that he only mounts those with red faces.