How late could someone in the camps present powers before they were deemed non-super and adopted out? Did any of the rejected children then manifest powers in later life?

emeralddodge:

The very latest is around age six, since there are systems in place to help children find their powers. Even really subtle powers, like Eleanor’s, would be discovered early. 

Very, very rarely they’ll get it wrong. In that circumstance, they evaluate on a case-by-case basis. A child with a combat-worthy power would be returned, but a non-combat one wouldn’t.