A lot of it depends on the individual horse and what it has been used for.
A maiden mare will normally have a difficult time at 18 to "take". But, if the mare has been bred on a (semi)regular basis at 18 there is no problem breeding them. (generally) I have bred older mares, but you have to make extra sure that their food intake is quality and sometimes help them along with probiotics so that the dam and foal get all of it's nutrients. And continue that nutrition while foal is on the dam.