The short story 'Wolf at the Door', by Mike Lee, described the Thirteenth Great Company as having been formed around warriors who had fought alongside Leman Russ for many years before the Emperor's arrival. None of them was "... younger than twenty years."
They were warned that the process of becoming Astartes would kill them, but none of them would be dissuaded. Out of the hundreds of the men who had comprised the companions of Leman Russ, "almost two score" survived.
"Ever since, the other warriors of the Legion referred to the Thirteenth as the Greybeards. The members of the company, however, called themselves the Wolf Brothers."
Kor Phaeron is another notable non-Astartes, but the Dark Angels seem to be the only Legion to have introduced such warriors in significant numbers:
"[Sar Hadariel] had been chosen to join the Dark Angels Legion by the Astartes, but in common with a large proportion of that initial intake, he had been too old to benefit from the implantation of gene-seed.
In its place, Hadariel, and others like him, including Luther, had undergone an extensive series of surgical and chemical procedures designed to raise their strength, stamina and reflexes to superhuman levels. They were taller, stronger, and quicker than normal men, but for all that they were not Astartes. They never could be."
I find it interesting that this was qualified for the Dark Angels... but not other Legions. There are a couple of ways to go about that:
1. The original fluff holds that the Legions were not all fielded at the same time: Alpharius, for instance, was found right as his Legion was being introduced. Under that scenario, the Legion that was the oldest - and thus the one that would have been fighting the longest - would need the most replacements. Thus, "stop-gap" semi-Space Marines might make sense until Caliban's recruiting got going. The newer fluff, though, (see 'Betrayal') holds that all twenty Legions were founded at the same time... albeit in regiment-size units (of "hundreds" of warriors) initially.
2. The Lion could very well have been intent on introducing the Calibanite element to the First Legion. He wanted the Order's traditions, training processes, and style of leadership to be incorporated by the Legion he inherited. The only way to ensure that would be to ensure that the best of the Order's heroes were represented somehow. That way, the actual adolescent recruits from the Legion's homeworld would have leaders and mentors who would bring them up in the same way.