WELCOME TO HT FROM MONTANA...
not a stupid question...theres no such thing!! A horse is never to old to 'train' I like to call it training

instead of 'breaking' ....I would rather have a trained horse than a broken one

I would def suggest you get a trainer onsite...as you may think she was only used as a broodmare...but she may have some bad habits and you may need a professional to help you out. Also, get a vet out to check her over...as she may have some lameness issues and that is possibly why she hasnt been trained yet.
Then you just start them out like they were 2 year olds...or weanlings...if it wasnt halterbroke yet

IMO...I think the horse is def trainable
