I am wondering the same thing.
I am looking at a Friesan whom I gotten to know well, and he's about the same age, and nobody's quite sure what or IF he's even been trained at all.
I would think they can be trained almost anytime during their useful years, but I might guess they may take a little longer, what with developing their current head-sets. You might run into some vices or habits that first will have to be dealt with.
Get a vet check to confirm her soundness and basic structures, but basically at any age after 5 if the horse is un ridden then you have a good chance of the horse having fewer soundness issues. The only thing I would be worried about with a horse this old is being "past her prime" and possibly breaking down faster because her body was not conditioned towards undersaddle work when she was younger.
I would go about it the same way as breaking a young horse and really make it a positive experience for her. Don't just hop on - condition her for it.
Who knows she could turn out to be a very good under saddle horse.
If I were you, I'd start her out the way you'd do with a young horse and eventually, once you've got her under saddle, try a little more than you might try on a younger horse's first ride. It might turn out she was ridden once or twice and knows the basics.
We bought a little MFT pony last year and were told he had no idea how to be ridden, but we didn't even have to teach him - once he was under saddle again he knew every last thing except how to canter; which is.. weird.
I would act like she's a young horse (I mean, she doesn't have the knowledge under her belt like an older, more experienced horse), & go from there. :) Gradually work with her- hey, she might not need much training if she's good with the basics! Just gradually train with her, & if she starts out good you can do a bit more.
I agree with starting like you would a young colt, but do not belive that 12 is past her prime LOL. Depending on what type of riding you will be doing with her, I would not be too worried about her conditioning, this will come with starting her.