Thats because they are. Spartan III's where created when the UNSC started running low on spartan II's, III's are just cheaper to make, plus i don't think Halsey was in charge of the Spartan III project. Halsey was in charge of the Spartan II project and was acctualy kinda like a surrogate mother, i guess, The II's typicaly refer to her as "mom" fo this reason i guess. Since she had a connection with the II's she didn't really care about the III's and is why she was more intrested in Jorge, because he was, in a way, her son like so many others. Jorge, incase you don't know, is a II just like Chief. And about that whole, them looking younger thing, Spartans typically see action when they are 12 years old. Jun was put into action earlier then most, for this reason, he has a screw or two loose, and has had a number of phsyc evaluations, and may suffer from post tramatic stress disorder. Jun is a III same for the rest of noble.