I think it not only varies from breed to breed but also climate conditions. We had an EXCEEDINGLY mild summer here. I mean, I'm still waiting for my tomatoes (which have been in the ground since March 9th!) to get ripe..... vines are heavy with fruit, but it hasn't gotten hot enough here in central California (where about a quarter of the country's tomatoes are grown) to get ripe on the vine. We're having to pick them half-green and finish them off in a paper bag.
That being said, I think the altered climate all over the country has caused a lot of chickens that would normally start laying young to wait a while. Leghorns are known for laying their first eggs at 15-17 weeks, but my girl was 21 weeks before she dropped out her first egg! That's old for a leghorn. All my birds, which hatched in April, have had good growth; but aside from the leghorn, NONE of my other birds is showing any signs of being anywhere close to being ready to lay, and they're all 22+ weeks except the EE who is 26 weeks! No combs, no wattles, no egg song, nothing. Faces are kinda-sorta pink but not even CLOSE to being mature enough to lay. I have a barred rock, a Rhode Island red, a welsummer, an Easter egger, and a leghorn.... the only one mature enough to lay is the leghorn, though the rest are all technically OLD ENOUGH that they SHOULD all be laying by now.
I didn't think I would ever get any eggs and started to wonder if my birds were duds. But 1 week ago today, our Felipa (leghorn) blessed us with our first egg & has been consistent ever since. Now we are just waiting on everybody else.