I didn't say an adapted biped can't reach 15 feet, but a modern human with a pituitary tumor cannot. Which is what the OP suggested. There are too many health problems associated with 9' giants of Wadlow's type for bronze age medicine to overcome, much less 12' or 15' giants.
>You don't know that. Even in antiquity, with Iron Age medical care, men commonly lived to 100 years, with the absolute limit, as placed by the Etruscans, at 110 years
I'm not talking about the lifespan of a normal human who happens not to develop any terminal illnesses, I'm talking about the lifespan of a giant with a pituitary tumor. At the extremes they have too many medical problems to live anywhere near a normal lifespan. Wadlow could barely walk, he couldn't feel his legs so he constantly developed blisters (this is what eventually killed him by the way), and like all extreme giants he had heart problems because his heart was enlarged and couldn't handle his pumping blood throughout his enormous frame.
Keep in mind these are the problems with a 9' giant. Now imagine the medical problems of a 12' giant. He would be a third taller and probably at least 2 times heavier. There is no way someone like that is going to survive in the bronze age, unless he is either of a subspecies that is adapted to growing that tall, or conditions were somehow different.