Often when someone wants to get a foal out of that good old mare they've been riding for 10 or 15 years, they meet with failure in that mare having a successful pregnancy. It's sometimes assumed that, well, she's just too old. I feel there are other factors than just age that impact a mare's fertility.
It is true that when mares reach the age of 15 or so there is a relative decline in fertility. But I have observed that if mares have been being bred and producing foals from early in life they maintain a higher level of fertility than mares that weren't attempted to be bred until their teenage years.
The 15-year-old maiden mares are often problems to get in foal. They still cycle normally, meaning that they produce a follicle and ovulate on a regular basis, but don't get pregnant. I believe the lowered fertility rate is based on the uterine environment not being receptive to nurture a fertilized egg and establish a pregnancy.
It is often assumed that these mares probably have an infection involving the uterine lining. I believe there may be more bacteria in the uterus on these mares, but think the bacteria are a symptom of a more basic problem involving the uterus. I find when I do an ultrasound examination on these mares when they come into heat there will be fluid in the uterus, even before they are bred or inseminated.
I correlated the presence of fluid in a mare's uterus with lowered fertility. The presence of fluid is due to the failure of the muscle (myometrium) of the uterine wall to have the proper tone and contractions to clear debris out of the uterus. This stagnant situation is a symptom of atrophy of the normal uterine mechanisms that are favorable to nurturing a fertilized egg and establishing a pregnancy.
On the racehorse broodmare farms where the mares have been having foals since they were 4 or 5 years old, I haven't observed the prevalence of this syndrome as much as in the 15-year-old maiden performance mare.
Now, let me remind you that what I have described is a trend only, and doesn't mean you can't get that good old mare in foal. There are some treatments that can be applied to help these mares get in foal, but in general you should be prepared for lower than normal fertility in the aged maiden mare.