Despite your constant notation that these young bulls should, by all accounts, not be successful breeders at young ages in zoos, you constantly seem to be proven wrong on that. A zoo environment is far different from the wild, and these animals, along with being developmentally far more advanced than wild animals at the same age, are also in, like you said, somewhat unique social situations, where a young male is the only possible option for a breeding bull.
With a little light looking, some young sires in "recent" years that prove you wrong are (at conception) for Asians, Bancho at 6, Kosala at 6 years 10 months, Winner at 7, Kiba at 7, Gung (Luk Chai's sire) at 7, Mac at 8, Romeo at 9, Emmett at 9, Albert at 9, Upali at 9 years 8 months, Doc at 9 years 10 months, Johnson at 10, Chanda at 10, Chang at 10,Assam at 10, Timber at 10, Thai at 10, Gajendra at 10, Po Chin at 11, Raja at 11, Ramon at 12, and Sibu at 12. That's 22 different animals from a number of different lines, on a number of different continents, all successfully siring offspring at young ages, and pretty much all of them aside from Albert sired their offspring through natural breeding. I think its fairly safe to say an 11 and a half year old bull asian elephant in a zoo environment can be generally well regarded as having strong breeding potential. They might not be anywhere near physically mature at that age, but they are most definitely sexually mature.