There is a few tricks for Cornish X to reduce the amount of leg problems.
Place the food and water at opposite ends of the cage, pens, coop.
Keep raising the food and water so they have to stand to eat and drink.
Put a few aggressive standard heavy breed broiler in with them. The slightly more aggress birds will make the Cornish get up and move more often.
Don't have feed available 24/7, use a 12 hour cycle for food. Like 8 in the morning to 8 at night have feed out for them. The rest of the time take the feed out of the cage, pens, coop.
The age you butcher can depend a lot on how the birds are cooked. For fryer you butcher when the bird is at the end of its juvenal period. For a standard Heavy breeds the butchering windows that is usually at 12 weeks or 6 weeks for a fast growing Cornish X. If you feed 24/7.
Some hatcheries sale a slow and fast growing Cornish x, use the slow growth ones, the butchering window starts as fryers at 10 weeks.
A lot on when the butchering window opens depends on the breed of the bird, how the bird was fed, and how it is entended to be cooked. Most of the advice given in this forum is for frying the birds.