When they start throwing a lot of slugs, still borns, deformities, etc. is what I suggest. Also consider how fast they recover and fill back out after giving birth. If recovery of body mass starts taking a long time, I would stop breeding them. They'll usually be fully grown by then, like, maximum size for their species, and will only shed 2-3 times a year. That's when you know your snake is getting old.

When to stop breeding is really your call. You'll just know when it's time.

On a side note, some people say 10 is old, some say 13 or so is maximum age but I had a male concinnus that lived to be 18, and a female that lived to be 21.