Pullet combs can begin turning red months before they begin to lay - I have a wheaten ameraucana pullet that is 39 weeks old - her comb has been bright red for months now, but she hasn't begun to lay yet.
19 weeks is very early to begin laying; the average time is 24 weeks, and you most definitely do not want to rush it. I do not feel layer feed until I see the first egg. Why? Because if they start laying too early, you often have complications later on such as internal laying or egg binding. So - as much as you want those eggs, don't rush it.
As far as breeding - IMO, they learn from watching others, so if you have adults that are breeding near enough for them to see, they could be mimicing what they are seeing. It's a bit unusual for them to start his early, but not completely out of the realm of "normal" if you know what I mean!
Many folks say that they will not breed until the pullets are laying - that's never been the case at my farm, and not as yours, either, it would seem!
Enjoy your chickens!