I think the Ad stating the horse that's bombproof needs an experienced rider, is because that horse's original owner most likely was extremely experienced and worked with that horse and trained it to become bombproof...
Honestly though sometimes age has to do with bombproof, sometimes temperment.. But really I don't think you'll honestly know the horse's temperment until you have one, yes someone who is selling may say all these great things and whatnot, but there's a reason they are selling whether they can't afford the horse, or really maybe downright need to get if off their hands due to temperment.
When I was sold my horse, practically given to me for free, sure the old owner told me yes he's a good boy and blah blah blah.... But honestly most react differently to their riders... My horse in the beginning, being moved to a whole new barn, with a new owner, new horses... Took a long time to get used to me, and because I'm not an expert, but maybe barely intermediate rider, He picked up on it and had spooked and thrown me a couple times... But now that he has gotten used to his new home and me has his new owner, He really settled down into the good boy his owner told me he was... Honestly for awhile when I first got him I thought oh wow what did I get myself into! but he calmed down.
Temperment is a interesting thing, it sometimes really does depend on the rider and riding experience because If you don't know what your doing and the horse is used to riding pleasantly with an experienced rider, he might get a little pissed if you're hanging on his mouth or your legs are flailing around.