1. How are they selected? Are there any tests?
The life of an Inquisitor is nothing but countless test. To live the life of the Inquisitor is to face constant test from all sides. You are constantly being scrutinized about everything you do. The higher ups will fear you and the lower orders will try to replace you constantly.
That aside Inquisitors are picked out of populations just like marines but for different qualities. They look for shrewdness, courage, and faith. If an inquisitor likes what he sees he will take the person sometimes with permission sometimes with out( whos gonna tell an inquisitor no after all). The person then lives in the retinue until the Inquisitor believes that he is ready to become one himself. he will make a request to his higher ups and they will issue the person test after test each testing a different virtue of the person.
2. What the average age?
20-500 with juvenant drugs. the younger they are the quicker they can be brainwashed
3.(fluff-wise) could a space marine be part of a retinue?
yes according to fluff the inquisitor can call upon ANY resource in the Imperium. From your toothbrush to a Land Raider (sometimes he doesnt even ask unless it concerns Space Marines but thats only to let them know that he is takeing their stuff and their is nothing the Space Marine can do.)
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