How long before cornish rock chickens lay eggs?

Cornish Rock chickens are a cross that is bred solely for meat. They normally don't live long enough to lay eggs. They are bred to grow too fast so they are prone to have heart attacks if allowed to grow much past broiler stage. If they don't die they will be crippled due to gaining so much weight while their bones are mostly just gristle. It would be cruel to allow a cornish rock to live past broiler stage.