KT, There is nothing wrong with getting an older horse like this- our oldest horse will be 26 in January, and he's as sound as can be. He's a retired polo horse- my sister plays polo as a hobby. As long as your new mare gets regular visits from the vet ( especially for dental care, because this will help to ensure that she can eat enough to maintain her weight and health) and is fed a good diet which is designed for horses her age, there is no reason not to buy her and make her part of your family. My suggestion to you is to have your vet come and give the mare a checkup after you bring her home, and ask him or her for recommendations about feeds, hay, and medical and dental care. This is also the time to ask other questions, such as those concerning foot care, immunizations, parasite control ( have your vet design a program for your horses which will keep them both in shape and as free of worms as possible- not only will this improve their health, it will help reduce the amount of money you need to spend on feed, because your horses will use what food they get more efficiently) as well as other issues, such as end of life plans, for example. ALL horse owners who have older animals need to have some type of WRITTEN plan in place which addresses end of life concerns for their horses. This is especially true for those persons who own horses which are insured- most mortality insurance companies require that consent be obtained from them prior to euthanasia of an older animal, and that an autopsy be done afterwards to determine the final cause of death, or they will not pay the claim. Getting to know your vet now, and allowing him or her to know your mare, will help you enormously later on- and you can make final arrangements in advance, just like you would do with your own funeral. This will help to ease the stress and pain you will feel when you finally have to say goodbye- which you will at some point.
As far as feeding your mare goes, there are any number of special feeds on the market which are designed specifically for horses her age, and which have the nutrients and levels of fat and vitamins that older horses need to stay healthy. One of my favorites is Equine Senior, which is made by the Purina company ( the same people who make Dog Chow) and is formulated to be easy for an older horse to chew and digest. You may also want to look into feeding your mare alfalfa pellets ( which can be soaked in water to soften them) instead of long stemmed hay- ask your vet about this. We feed our horses a mixture of soaked beet pulp, Legends grain mix ( this is a wonderful grain mix which is relatively low in sugar and starches, but which has plenty of protein and other nutrients horses need) alfalfa pellets, and a special type of grass hay called Teff. Teff differs from regular grass hays in that it has more protein, and it is also drought resistant, which means that it can withstand long periods without rain and still grow to maturity. Our horses love it- and they all do very well on it. ( We have 11 of them on the farm I live on- 5 of our own and 6 boarders.) This program works for us, but your circumstances are probably different.
Lessons for you and your kids are a wise investment- they will help you feel comfortable and safe around your horses, and they will help to keep your kids safe as well. Make sure that your kids wear helmets EVERY TIME THEY RIDE, no matter what- and please, set a good example and do the same thing yourself. Helmets SAVE LIVES- and there is no getting around that fact. Riding without one is just NOT SMART- ANY horse, even one as well trained and as old as this mare, can spook, buck, bolt, spin, rear, or whatever at ANY TIME- and it only takes a second or two for disaster to strike. I have met and seen far too many well meaning parents who think that helmet use is for sissies and greenhorns, or who think that nothing bad will ever happen to them or to their kids if they go without a helmet. This kind of thinking is not only irresponsible, it's downright DANGEROUS, and it can and HAS gotten people hurt and KILLED. Riding is a wonderful sport, but it is also a risky one- and when parents let their kids ride without helmets, they are taking the kids' lives in their hands- and putting them in serious danger. Along with helmet use, there is the issue of proper footwear. NO child or adult should EVER ride in shoes which have no heels ( such as sneakers or athletic shoes), and it goes without saying that NO ONE should ever ride while barefoot or in open toed shoes such as sandals or tevas. Long pants ( jeans are just fine) are also a requirement, regardless of the time of year- riding in shorts or a bathing suit not only looks bad, it's uncomfortable and can be dangerous. Riding in sneakers puts the rider at risk of being dragged under and possibly killed by the horse if the rider should get their foot caught in the stirrup as a result of a fall. English saddles DO have breakaway stirrups, but since the great majority of riders generally ride with the latch on the stirrup bars ( the place where they attach to the saddle) in the closed position, that negates any possibility of the stirrup coming off in a fall, as it was designed to do. Western saddles are even worse in this respect- they DON'T have breakaway stirrups. Getting your foot caught in a Western stirrup because you were riding in sneakers and fell is a recipe for disaster- and I have seen it happen. So be smart with your kids, and insist that they wear their boots or sturdy shoes with a heel of at least 2 inches when they are riding. You'll be glad you did the first time one of them has a fall, believe me- and it may just save you a trip to the ER.
Good luck to you- I hope this helps.