A while ago, I composed a bit of fanfiction where an original character is adopted by Queen Cersei from Game of Thrones after she outlives her natural children, and she makes him her baby. (It's below, if you're interested):
https://www.deviantart.com/9catstrek...Pt-1-881866333
But we don't have to use Game of Thrones, especially since you told me you hardly know anything about it. The concept I'd like to use is where a queen, who is unmarried and childless for whatever reason (be it she is barren, uninterested in romance, or prefers women to men), decides to take in a teenager or young adult man as her child.
We could combine that with your idea of the babied individual being perceived by others as a baby, and the queen's adopted son could be seen as a baby. How's that sound?