It's impossible to say when they will start. Each chicken is an individual. I've had some start at 16 weeks. I have some hatched April2 this year that have not started yet. Heredity has a lot to do with it. If you know the age the flock they came from usually starts, you can get a fairly good clue. But even that is not a guarantee, just a clue.
Will they put it off until spring? Maybe, maybe not. I had some that started laying early December last year while the days were about as short as they get. They were about 5 months old. But a few of that group waited until February when the days started getting noticeably longer. I do not use light to extend the length of daylight.
I keep oyster shell on the side and feed grower when I have a mixed age flock. The ones not laying will sometimes eat some oyster shell. They don't eat a lot, not enough to hurt them, but they will experiment with it. The ones laying eat a lot more.
How can you tell if they have started? If they are free ranging, it is possible they are hiding a nest from you or that something is eating the eggs. When a hen is laying or getting ready to start, her vent is large, damp, and soft. If you look at the vents and they are tight and dry, they are not laying. Looking at the vent is not a guarantee they are actually laying but you can get a pretty good idea if they are close to starting.
Most wild animals will not eat every egg that is laid every day. It's possible but that's not a normal pattern. If every egg is disappearing every day, I first suspect a family dog that has learned where to get a snack or a human. You might try putting an egg out there and see if it disappears. Warm it up to outside temperature first.
You can lock the hens in the coop for a couple of days and see if you get any eggs. That does not tell you if they are hiding a nest or if something is eating them but at least you would know they are laying.
Good luck!
