Space Marine recruitment inconsistency

So long story short, I’ve read a few novels and comics that go into detail about the recruitment process (pre-surgeries) and most official lore seems to state aspirants go through trials at like 12-14yrs old. So children. But a bunch of media clearly depicts aspirants as buff dudes in their 20’s or 30’s. Heck the hairy bodybuilders in the image attached go on to become Black Templars. But if that’s possible, why do they make such a big deal about older figures like Kor Phaeron and Luther being unable to fully transition into astartes. Thoughts? Is it just lazy storytelling? Some sort of exception based on gene seed? Whatcha reckon?