Both are still laying and the older one hatched and raised 16 chicks successfully this year. She is very experienced and in all these years lost just one chick.Those are good ages. Do either of them still lay eggs. The eldest here laid a couple of very thin shelled eggs this year and promtly broke them leaving the nest. She's ten years old now.
Did you try to supplement her with extra calcium+VitD+VitK?