Agree with shygirl, absolutely no reason why a mare of that age shouldn't breed, I have had a first foal from a 16 year old mare, but a vet check first is a good idea - external, internal and a scan, to check for good reproductive conformation, and also any age related changes which might limit her chance of getting in foal. Like humans, the age at which these begin vary greatly.
Not sure if I am reading right about the shetland, but she is 6 years old and about to have her 5th foal? So she was covered as a yearling? Really hoping I have read that wrong!