If you have hatchery stock, they most likely will not go broody. Hatcheries have bred that trait out of many of the breeds today. Age likely has nothing to do with it. I've had hens go broody a month after they started laying.
I've never had a hatchery Barred Rock or hatchery RIR go broody, not ever. I have had one of my heritage show quality type BRs brood, though.
Your "Americanas" if you got them as hatchery stock are Easter Eggers and since they can be quite varied, being of mixed genetics, it's impossible to say if yours are the broody type. I have had one of my true Ameraucanas brood, however.