[@Lord Coake][@Roosan][@Jbcool] What Jb says is accurate: they could be altered to become more powerful than normal humans in canon, just not as explicitly superhuman as an Astartes is. That was actually the basis of Luther's fall to jealousy, and thus Chaos, so take that as you will. Anyway, adults can become Space Marines in this game thanks to Emperor biology changes, with the advantages of more experience and less chance of death than children and teenagers, but a subsequently slower rate of individual conversion due to puberty not being exploitable for having already passed.