THe Duke of Kent was a Counsellor of State from the 1950s on. There was a dearth of senior adult Royals who could serve as Counsellors in the 1950s and 1960s and so the Duke and his sister Alexandra both became Counsellors of State when they turned 21. I think they, along with their uncle the Duke of Gloucester, Princess Margaret, Prince Pilip, and the Queen MOther were the only Counsellors of State until Anne and CHarles came of age. Maybe Princess Mary, the Queen's aunt, served as well? I can't recall.