Astartes are forced to recruit kids between 12-14 years. Basically the onset of puberty, any older and the odds of their bodies rejecting the implants and extra organs increases exponentially so it's not seen as worth the effort. After some years of further training & numerous surgeries they are made into scouts at roughly the age of 19-24. I included an age outline in the CS. 

Applicants are not vat grown but picked from the youth of a planet, usually a pretty shitty planet either a deathworld or the bowels of a hive world. In the case of the Salamanders they exclusively recruit from the surface of Nocturne. A pretty shitty world wracked by earthquakes, tectonic shifts, volanic eruptions and covered in ash wastes. Barely livable but still livable.