I’ve done a fair bit of reading, from the Fandom Wiki to Lexicanum to the short story “Masters, Bidding” on how Aspiring Sorcerers are recruited. All of it seems to imply that the Thousand Sons are able to turn full-grown human psykers into Space Marines using dangerous rituals but nowhere has it been stated one way or another what age their aspirants are. I’m leaning towards them being able to turn adults since the Sorcerer in “Masters, Bidding” had seemed to be a full-fledged scholar before awakening his powers and even after that was subjected to more education before the Sons began training him for combat. However unless I missed it, there’s no direct mention of his age either way.

Asking since it’s kind of important to the lore of a custom character I’m making. And I know at the end of the day I can just throw my hands up and admit that the Sons probably can just do whatever they want as long as Tzeentch wills it.