When are mares most fertile?

Fertility in mares peaks at about 6 or 7. It drops off significantly at about 15, and most breeders don't breed a first time mare at 13 or older (you can try, but the success rate tends to be very).

However, many mares, especially ones that have been bred frequently, can keep producing into their 20s. I've even heard of a mare foaling at 30 - I assume it wasn't an intentional breeding at that age...