When are cockerals fertile?

[AfewBricks]

I have a 10 week old cockeral whose acting like a rooster in every way. He's been crowing and mounting my bantam cochin for 2weeks already. Now he's taking on the 4 mth old full sized pullets. I've had rooster chicks before, but nothing like this.

 

[Judy]

That does seem young! It would be interesting to check an egg or two for the bullseye, maybe a week or so after this started.

Just in case you're not familiar:

https://www.backyardchickens.com/t/16008/how-to-tell-a-fertile-vs-infertile-egg-pictures/0_20