My hens all had fertile eggs even at ten years old. I know that Porter's switches his hens out after 4 years old and his toms after 3 years old. I suspect is the problem is toms more than hens.
If your hens are getting supplemental light, it is normal for them to be laying now. Mine do not get supplemental light and will not start laying until mid March.
You are farther south than I am which also can make a difference.