Most chicks start this process somewhere between 18 - 24 weeks of age. At this point a most people have a hard time telling them apart from adult birds.
One of my (not so) little hens Dusty, had a false start at around 20 weeks of age. She laid a single perfect egg. WOW! Small (as most pullet eggs are, but perfect. About a week later she laid another but this time it was very thin. A few days later she laid 2 and both were very soft. Neither of them were in the nest and both were broken. These are sometimes known as rubber eggs. Chickens can be culled (nice word for made stew out of) for continuously laying them. She laid 2 a day for another week and then...not. another. egg. Time passed and I was very concerned about her.
Was she egg bound? I tried soaking baths, apple cider vinegar, adding extra protein, extra calcium, all sort of things. You see I was not concerned about the eggs. This particular bird is my girls pet. The other birds are egg layers, but this one? Not so much. This one is much more of a pet. She's a lap bird. I wasn't sure just how long she could go without laying again. 1 week went by and all was well. 2 slipped by and this was when I started to worry and the home remedies began. 3 went and that is when I started to think that I should possibly start looking for a replacement. Turns out right now isn't a good time to buy this type of bird (a Brahma) all the hatcheries are sold out!!! But, in my search I also noticed something else. She's not acting sick. After 3 weeks of not laying if she had a problem that was keeping her from laying she should be showing signs of some kind of illness. Upon further inspection I found that a few of her wing feathers had been replaced. Crazy chicken was MOLTING! Pullets (young chickens) aren't supposed to molt like that. But, she's a very heavily feathered bird and it is summer in Florida after all. Who could blame her? 4 weeks. Still no egg.
Then, just as I'd given up seeing an egg until Spring guess what I got? What?! You know the answer to this because I posted about it already! 4 out of the last 5 days I've had 3 eggs in the nest. I'm proud to say that the 5th day the third egg that was missing wasn't Dusty's egg.
How can you tell when a bird is getting ready to lay an egg? There are a few ways to tell, but the easiest way to tell is by looking at the development of the comb, ear lobes and wattle.
This is dusty:
Here she is at 18 weeks of age. Just prior to her false start: