Luther and Kor Phaenon were both too old when they met the Astartes so they have been genetically altered and buffed up but not to the degree as a regular Astartes.
One example from /spoiler\ the First Heretic has Kor Phaenon asking Erebus, a full Astartes, how the battle between Lorgar and I forget which other Primarch is going because they're fighting far away and at a very fast pace.
Lukas is perhaps the only Astartes who is still attracted to women post-operation for unknown reasons.
Young ten year old boys who impress the Astartes are worthy in the sense that they are eligible to partake in the trials to become a neophyte because to endure such trials you have to be aggressive and extremely fit.
So in short, you have to be in your early teens to become eligible to take part in the trials to become an Astartes as the organs alone will take a while to adjust to your body.
Last edited by Malus Darkblade; 03-21-11 at 11:05 AM.