That really depends on weather or not she has been bred before. Maiden mares are a little harder to get pregnant and tend to have a harder time keeping a pregnancy. At 18 your mare would be the older side of a brood mares career. Older mares have a harder time keeping weight on (as a general rule) and add to that a pregnancy and the demands of a foal. I guess it would depend on why you want a foal? Do you want one just because you love this mare so much? Or do you want one because she has amazing lines that you want to propagate?
I'm no breeding professional but 18 is going to VERY risky for both mare and goal.
Other things to consider...
Horses are selling for a couple hundred dollars. Way cheaper then breeding a foal. Vets and trainers don't work off good graces. Do you have the time, energy and skill to devote to this baby? What about the money for extra food for your mare? What has your mare done with her career? What's her conformation like? Show record? Health? Posted via Mobile Device
Have an equine repro specialized vet check her out, they can help determine her fertility and overall health as to the potential of carrying a foal to full term. Many 18 year old maiden mares can still be bred and carry to full term without problems, but the care put into them is more than a younger mare would need. Careful diet and exercise plan to keep proper weight and good physical condition are essential.
You also need to be critical of your mare's faults, conformation and genetics. Be even more critical of stallions, what they offer in conformation, disposition and genetics. Study genetic diseases known to the breed and make sure any stallion you consider is tested for them. Also know the frame overo status of your mare if you want to cross to a stallion that is a carrier. The purpose is to roll the dice in the breeding gamble to go in your favor. There is always the risk of losing the mare and/or foal with everything else perfect. You want to decrease any chance of a dead or unhealthy foal by careful selection of stallion genetics. Conformation is very important, not only for being capable of the discipline you want, but also for long term soundness.
Another important thing about breeding is how marketable the foal would be if you ever had to sell it? Can the foal be registered? How desirable are foals from the stallion? No one can foresee the future, even if you have a plan to keep the foal forever, you never know what may arise that could result in selling your horses.
It all really depends on the mare in my experience. We had a mare that was bred at 16 and had the foal at 17 with no issues. It was her second foal, although the first one had been about 10 years prior. I've known breeders who use broodmares into their 20's and still have healthy foals. If you're really unsure, ask your vet to do an exam. They would be able to give you a pretty good idea as far as health, risks, etc.