Sorry OP, but I really wish people would stop posting these threads.
Horses can live until 30 in good care, easily be rideable until they hit 20 and maybe slow down a bit after that (but I do know many horses that make it until late 20s and still ride pretty hard). Past that you should decide, there are trade offs with a horse of any age, generally long term soundness vs experience. If you are willing to get a green-ish horse and want to keep it for 20+ years sure get a younger horse.
The reason people suggest "older" horses (mid teens usually) for beginners is because those are the horses that have been there done that. And, after riding said horse for 5, maybe 10 years they will know if they want to step up and get a (maybe) greener horse that is more athletic and capable of high stress events (like jumping)
"I don't think he ever gave a thought to other people's opinions, which was just as well because they were often unkind."
-- James Herriot, All Creatures Great and Small