Question About Black Buck-At What Age do They Reach Breeding Ability

Was wondering how old a black buck male needs to be for breeding purposes? How long would their horns be at this point? Am only asking about horn length so that I don't buy animals too young. Need to buy a couple younger males for breeding. Thanks.

Here's a link that gives a godd overall sketch of blackbuck biology http://www.allaboutexotics.com/exotic-species/blackbuck-antelope/