Are you sure of their age? And are you sure of their breed?
Most sex links start laying very early, like at 16 weeks.
The combs will turn bright red before they start laying. And mine also will hunker down in the "breeding position" when I go to pet them right before they are about to start laying.
Could it be what you feed?
I feed whole grains mostly and I make a mash and my Black Stars started laying huge eggs at 16 weeks old. Mine also free range. Environment has a lot to do with it, too. They like to lay in a dark nest box that is covered and where it is relatively quiet. Well, not always as sometimes I find an "oops" egg out in the dirt in the courtyard from my Faverolles usually.