Ha - that's a loaded question!
Well, any horse can be trained at any age, but the longer a horse has been left to its own devices, the more bad habits you may have to deal with. You are also going to have to devote more time and energy to the task. It's not impossible - an old Morgan we had when I was a kid had only been trained for driving and pulling, and at age 20 he took on a new career under saddle and ended up loving it.
Also, when they say she is "not broke", that can mean a variety of things to different people. Is she not trained under saddle? Is she halter broke, at the very least (I would hope so...)?
Here's my personal opinion (because I can't keep my opinions to myself...
) - I'm always suspicious of anyone who is breeding a horse that has not proven that it can perform a purpose. When any animal is considered for breeding, the breeder should be devoted to passing on traits other than color and a fancy pedigree. To me, color is irrelevant, and so is pedigree if the horse is obnoxious, mean, dishonest, bites, kicks, etc., etc.
However, there is always the posibility that she is a wonderful mare who was injured in some way and she is now a brood mare because of that. One just never knows...