Were you told that they are laying? They are old enough to potentially be laying now, but sometimes a change like moving to a new home can put them off laying for a few days, sometimes a week or more. I've never had black sexlinks, but have had chickens that have laid double yolkers in their first year. It seem that eventually their system straightens out and they start laying single yolked eggs.
My Barred Rocks are almost 5 months old and two are squatting for me. In my experience when they start squatting that means they will lay no later than 1 week after squatting. So watch your birds. They will tell you when they are starting to lay. First comes squatting for you. Once they squat then that means you will have eggs no later than a week after squatting for you. Remember, first eggs can be small(called pullet eggs) but dont worry, as the hen matures they will become larger.so dont be disappointed if the first few are small or down right tiny. Its normal. They soon get bigger.watch your girls combs, when they become red they are close and when you go to pet them and they squat for you the time has come for eggs!!! Hope this helps and enjoy yoyr girls!!
Hey Dean, welcome to BYC. I'm assuming this is your first experience with chickens? If not, forgive me for making an assumption. BSL's are production birds and will typically start laying early. I do not have any production birds in my flock, but often get girls laying around 16-17 weeks. A lot depends on time of year, breed of bird, and their nutrition during the first few months. As PP said, look for squatting. They may also start singing the egg song prior to that first egg. Their faces, combs and wattles will get more plump and the red color will increase. Do you have the nest box(es) set up? (best to get them set up before that first egg appears.) They will start "playing house": climbing in/out of the nest boxes over and over again, scratching around, playing with the hay. Give them lots of bedding in the nest box. Be sure the box has a nice high lip on the front, (4" or so) and a perch in front of the box if it's up off the floor so the hens have somewhere to land prior to stepping into the box. You can also give them some golf balls, or any other thing that vaguely resembles an egg. You can buy wooden eggs at a craft store, plastic Easter eggs, ceramic eggs at a feed store, or use golf balls. I do a combination of Easter eggs filled with sand or pebbles wrapped in tissue paper, and then glued shut, and used golf balls. When I have pullets that are new to the laying game, I keep them shut in the run so they don't get in the habit of dropping eggs in the yard. (train them to the nest). Sometimes, those first eggs come as a surprise. You might find an astonished pullet looking at that strange thing that just fell out of her butt. And, it can happen anywhere. Those golf balls in the nest box give them an idea that "THAT is where those eggs should go!"
You'll find lots of good articles in the learning center to help you with your "chicken owner" education. But, don't be afraid to ask, and when you do, it's a good idea to start your own thread. That will get you more personalized information. You also might want to put your general location in your profile. That will help folks to give you information specific to your region. Also, check out your state's thread in the "Social_ Where am I? Where are you?" forum.