@TheSeriousJoke The organization doesn't corrupt everyone. They corrupt those who have the more useful powers. Like, oh, Orion Faust, the piano prodigy, probably wouldn't get corrupted, but someone like Vespira might, with her adoptive muscle memory.
Regarding adult Stars, it really depends on the nature of their power. Generally speaking, star marks appear by the time the individual hits puberty. If they can go undiscovered until they're adults, generally it's not worth the trouble to track them down. If they come to the facility of their own free will, or are discovered to have a potentially useful ability, then they will be taken in and given accelerated crash courses in like, controlling their powers, and their educations will be shored up. Then they will be offered positions working throughout the facility, perhaps as teachers, or guards, or tour-guides, or something else entirely (again it depends on the power.)
When Stars within the facility reach the age of eighteen, the more trustworthy ones (the ones with benign powers, or who have exercised impeccable control) are given a choice: Remain at the Facility and work to teach and train younger Stars, or go out into society. The Corrupted stars who reach eighteen are fairly few and far between, but usually they are kept as field agents or researchers (again depending on the power. Someone like Astraia would probably not become a field agent, but can you imagine Ursa trying to sit at a desk and do research?) For those who would be ill-suited to those professions (like Aquarius) well... Some do the darker deeds for the organizations spies and research groups. But for the most part, their fates are a mystery.
So, for the corrupteds who go mad, like completely-lose-control-of-their-abilities insane, there's like a basement sort of area, (totally off-limits to regular kids, of course), where they are held and attempts are made to rehabilitate them. If that fails, and it usually does, they're... well, I'll leave that up to your imagination.