You don't say where you're from but one thing to consider when hoping chickens will lay this year is the amount of light they get per day. They need about 14-16 hours.
They will get red combs and wattles when they get close to laying. Look for those first before the squatting (I think).
Somehow you can feel their vent (their bottoms) and feel if they are close. I have never done this so if you're interested, look it up on here.
Make sure when they do lay (or get ready) to either switch to layer feed or offer calcium (or both).
I think 15 weeks is probably a little young but if the lighting is right, you could probably expect eggs in a couple weeks. BUT every chicken matures at a different rate so you just have to be patient.
It seems like the magic way to get a chicken to lay is to post a "when will my chickens lay?" topic. Every time I read one, the original poster posts in a few days with pictures of the eggs.
Good luck!!!
OH- You can also use the search bar at the top right of the page to find out more about egg laying readiness signs.