Originally Posted by Malus Darkblade
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.
Lukas is perhaps the only Astartes who is still attracted to women post-operation for unknown reasons.
So in short, you have to be in your early teens to become an Astartes as the organs will take a while to adjust to your body.
All is well and good but I'm looking for a source that directly rules out the possibility of an adult Space Marine inductee. Right now everything points in your conclusions direction, but I'm clinging on to that 'ideally implanted at xx age'. In any case an adult inductee would recieve his first implant ten years after it could have been 'ideally implanted'. No doubt would the aspirant suffer for this. Imagine an imperfect marine with only half functioning implants.