It varies by chicken. Some roosters, especially young ones, will try to mate with anything around, regardeless of age and sometimes even sex. A lot of the time, it is a dominance thing. I've had older hens "mate" with younger pullets when no mature rooster is around. Many mature roosters "know" when a hen or pullet is ready and has some control of his hormones.
Some hens or pullets want to be mated. They'll squat for anything with spurs. But a lot, especially more mature hens, expect a rooster to behave properly. Dance, find them treats, and offer protection, keep peace in the flock by breaking up fights, the things a good rooster is supposed to do. Many adolescent roosters are not very good at any of these and just give in to the hormonal urge. Many pullets and a whole lot of mature hens don't like this and will resist. I have a mature hen and a fairly immature rooster right now. Most of the egg laying pullets will willingly squat for him, but the mature hen whips his butt if he tries to get fresh with her.
I could be really wrong, but I'd think your 20 week old rooster has his hormones fairly well under control and just knows the 16 week olds are not quite ready.
That the pullets willingly squatting for him, even if he ignores them, is a pretty good sign that they may start to lay fairly soon, but I had a 13 week old pullet that would squat for a rooster and she did not start to lay until she was 20 weeks old. Them squatting is a good indication but not a for-sure sign.
