I've butchered some at 14-16 weeks, they are small but are filled out nicely (pure Cornish) If they look like they could use a bit longer to get to the size you like, then let them wait a bit. Try catching and weighing them. If they have the hard feathering their size can be deceptive (they are actually much heavier than they look).What age do you process the Cornish and about what do they weight? I have some mixes that are 16 weeks tommorow, but they look like they need a couple weeks... but the weather is saying otherwise