It depends on the individual. I've heard as mares get older, they can be harder to get in foal if they've had a couple years off... But there are mares who produce into their 20's, too.
Maiden mares over 15 should be seriously evaluated before breeding. At that age their cervix will have started to loose its elasticity. It is possible to get her pregnant without problems, but have an absolute disaster at foaling when the foal can't get out.