Some of the longtime chicken keepers on here have noticed that the rooster's temperament is not really done developing until after a year or possibly 2 years. If you have a nice roo and he's over a year old, your odds are good he'll stay that way, but of course there are no guarantees with live animals.
I've taken that as my guideline, and have seen that it works for me. Everyone who was even a little human aggressive after 1 year old was already showing signs before then too, even if they were small or we weren't quite sure. We got really attached to our first roo, and it was heartbreaking to put him down. But he was not safe for the kids, so he had to go. Now, we don't name them until after a year old. They have band numbers if they need to be told apart.
We finally seem to have a nice roo, he's about a year and a half, going on two. We were starting with hatchery stock, and it took two breeding roos from two different breeds, and 25+ candidate roos that we hatched ourselves to get a nice one. Now, is he fertile? Will he hatch out calm chicks? Will hens he fathers lay a good amount of large eggs in an appealing color? All of these are still unknowns. But he's non-human-aggressive, a decent size for an egg layer roo, and very pretty, so we're willing to wait and see how he does this next year.