A 12 year old girl would almost certainly be maturing sexually, and as seems clear, Sapphire has the characteristics of a developing young lady. at 15, it would be all but impossible to pose as a boy. If she had not been posing as a boy all along, it would not be too likely that she could get away with it. But as everyone 'believed' or pretended she was a boy for so long, they could continue doing so. However, Duracell
& Nylon, being suspicious all along, would be more so as she develops adult characteristics. In the anime, Rock/Franz knew from the start that Sapphire was female, & there were some humorous things that occurred as he tried to force her to admit it.