Its kinda like people....As we get older things will stretch out but not goe back were it origianally started.
As things age the calogen in the skin depletes. Older mares have a higher chance of not carring to full term and if you have an older maiden, the bones might not open up as good causing a difficult birth. Age does this. Tearing is also very more probable in an older mare than a younger one.
Colic is also higher in older mares after birthing and can be a death sentence leaving you an orphan foal. I believe in keeping their system full of dry hay and give them all they can eat to keep the intestines from twisting when giving birth.
Breeding is a crap shoot and any of us that breed are taking a chance of loosing one or both. Even with the best of vet care this can happen. I'm also a fan of breeding big mares to small stallions. Draft breeds will have big heads and large shoulders and can make giving birth to a maiden light mare difficult.