I think it really depends upon the bitch? Storm was six when she had her second (and final) litter and was absolutely fine. She's very fit and active and had no problems at all. Pups all strong and healthy .
Thanks, good to hear from someone who has done it correctly,
Seems sensible that if a dog is in good health, is fit and strong, and has been bred correctly, six seems a good and sensible maximum for the Kennel Club to bring the limit down to.