You could offer them crushed eggshells.So, if they eat it when they need it.... What do you do when they will not eat it but their shells are thin? Is there something else that will harden their shells that they will eat. Note - They are only 2 years old.
If you add water to chicken food to make a mush, chickens usually think it's a treat. If you mix oyster shell and chicken food, then add water, the chickens might eat the oyster shell along with the chicken food.
You could sprinkle a little oyster shell on the ground, to see if they eat it better that way.
You could try a different brand of oyster shell--some people say their chickens like one brand better than another.
You could feed "layer feed" that is supposed to have enough calcium, which would mostly solve the problem, although a few hens might still need more calcium than the feed contains.
I don't know exactly what will work for your hens, so I'm just listing everything I can think of, hoping that one or another idea will help