Castration is the removal of testes before puberty. The testes are responsible for producing testosterone. The testosterone in turn is responsible for the secondary sexual characteristics such as the voice.
The testis need a temperature of 1 degree less than the body temperature. If the testes are removed, the hormone is not produced in the body and the boy remains a boy, without maturation of the external genital or the development of any secondary sexual characteristics. It is the same as a male being born without testes. He will still be a boy, but he can never become a man or reproduce.