A 13 y/o should be fine physically carrying a foal to term (or course, there are always risks involved) but she might have more difficulty getting pregnant than say a five year old maiden, because of reasons SR pointed out.
I suggest you get your vet out to do a breeding soundness examination and cultures before breeding. Your mare might have built up uterine fluid or infections (which would most likely prevent her from becoming pregnant) after all these years of being empty, and it will save you time and money knowing about it and being able to treat it before you even breed her.
A good horse is worth more than riches. ~ Spanish Proverb