I would ask your vet. As long as she has had proper nutrients and at a healthy weight (not over or under weight), she should be fine. I bred a 14 yr old maiden mare and it took three times. Pretty sure she's expecting since she hasn't been coming into heat, but will be getting an ultrasound :)
Proud Partner Owner of Me Too Radical, Top 10 PQHA Quality Improvement Program Futurity. Sire of AQHA, NSBA, and NRHA champions. Sweet, gentle, excellent mover, excellent temperment, and so much more. Standing in Indiana.