It is said that part of the transformation uses hormone treatments. If you dump enough testosterone into a teenage boy, you accelerate puberty including the physical changes to the larynx and vocal chords. You also very likely cause rage and psychopathic behavior as well as sterility; all traits common in Space Marines.
But you could probably find a few green, soft-voiced Guardsmen. The recruitment/conscription age on some worlds is fairly young.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."