Before you go raving about how the previous owner was a 'scammer' and a 'liar', you should probably consider the idea that they might not have known the horses actual age to begin with, ie, that the person they purchased the horse from gave THEM an an incorrect age and they've not known any better other than to assume they wre right.
Secondly, I agree with the other people who have said a horses age has nothing to do with wether or not they are broken/saddled/rideable or even handled! There are numberous farms out here that have completely unhandled 8 years olds, never mind rideable!
If your horse is sound at ground level, but spooks or 'misbehaves' when ridden, you should consider your technique. Do you just dive in there and climb on, or have you tried just leaning over their back and being lead around so that they get ued to the weight on their back? There are numerous 'methods', both tried and tested and completely insane sounding. Regardless, the key is confidence; if you're nervous, you're going to upset the horse.
If the horses is a 'rearer' and 'disobediant' only under saddle, I suggest calling out the vet or an equine chiropractioner to check him/her back for problems, such as a misaligned vertabrae or even just muscle knots, especially if she has been previously ridden without problems. Ensure that the saddle fits properly, and that the rider isn't too heavy (no assumptions here! Just suggestions!) Aside from physical problems or ill fiting tack, there is the possibliity of some psychological trauma that was caused whilst being ridden.
There is no need to put a horse down just because it's 'naughty' under saddle. There's always the chance that the rider and horse are simply not a 'working' partnership, and considering the fact she was apparently 'rideable and has been ridden in the past', and all other options above have been exhausted, this may simply be the case.