They would sit for 20 days maybe but very young hens might not sit the full 18 days anyway, after this if they go down on eggs then let them and keep the young. I have a very old cock I have recently bought, should his hen lay and they are fertile, I will keep them, late bred or early for next year. I paid good money for old pigeons if I get two pair at any time I will be more than happy with that.