i would say unless she has had a foal b4 u wann a do it asap as they get older it gets harder to get them in foal plus when theyer older there body cant recover or cope so well
I have known mares up to 17 years conceive and foal first time without problems, but I don't think it is that wise. You would get more knowledgable answers on this if you were to ask in Breeding.
Try the breeding section you will get more experience in there. The breeding age for a maiden will be determined by the indivdual mare because there are things like fitness etc to take into account. Your vet (if equine) should be consulted as the mare will benifit from being scanned beforehand.