Because Gaia/P4-11 is a newly re-discovered human colony, Genecia proposed to the delegates of the Aohu Accord (the most important conference in Gaia's history, similar to the Bretton-woods conference on old Earth) that minors, i.e.: children under legal age are ill equipped to handle conversion into the two major factions of humanity and social problems will emerge as a result of premature assignment.
The agreed upon benchmark was 21 solar standard years. After which, each individual human would decide if they wanted to become Corp, Incarnate or Unaffiliated
The assignment process was quick, provided free by the jurisdictional Civil Authority (most of the time the Megacorp that has ownership over the area of residence) and of course painless.
Now the very first assignment exercise was conducted en-masse with the cooperation of the standing militaries of the participants of the war. Crowd control was facilitated by soldiers. Orderly queues and tents were set up, and pre-assignment talks conducted by trained councelors and post-assignment debriefings were conducted in batches. It was a massive undertaking that took months. Without GENECIA's help, it might have even taken years as many identity records were lost in the war, family trees could not be traced and a huge amount of orphans existed because nearly an entire generation of fighting-age men were wiped out. This was PRE-transitional. And a lot of humans were fearful of the change to come and unsurprisingly more than half of the population chose to stay human and 'unaffiliated'.
Given that internet infrastructure was yet to be built, the true advantages of being a silver blood (i.e.: a Corp) was not fully realised. It was like having a smartphone with no 3G. So among those who chose to be assigned, many chose to be Incarnate. And those that did, were surprised to find that some already had latent Incarnate DNA (inherited from their original Earth ancestors).