That was a problem for me for a bit, although at earlier ages (starting at about 5 months). I didn't so much train them as the flock trained them out of it.
In my case I had a somewhat large mixed flock of 30+ chickens and guinea fowl that was 8 months old or older before I got the roosters as...
As others have said, it depends on what you want to do beyond being able to fertilize and hatch your own eggs.
If you're free-ranging your flock, my experience is you need one. I thought having guinea fowl would protect my hens but this had mixed results: the guineas were great at keeping hawks...