Stacey, I think it all depends on the doe and her attitude to being pregnant, and how well she recovers. For some it could be rather young, for others, well my 14 year old had twins, and she did not recover real well, but she is the one that had cancer.
We haven't had that old of a doe yet, but I think it depends upon the condition of a doe, how she handles her kiddings, like if she tends to get skinny after kidding I wouldn't breed her for a year or two(but she'd be milking in that time) From what I've seen with most breeders, they stop breeding at around 8-10 years of age.
Stacey, I've had my Bootsie since she was 6 weeks old and she is turning 8 years old in May, I am choosing not to breed her anymore after her triplet bucks this time. She has ALWAYS done great, easy kidder , excellent milker and gorgeous kids, I just feel that I love her too much to risk anything happening to her during kidding as she gets older. I have heard that goats will live longer if they are "retired" from breeding because some does bred too "old" have more of a chance of dying. I let her decide what she wanted last fall and I'm glad she chose Chief...otherwise I would'nt have gotten Teddy from her.