Avi K,
You may have misunderstood what I wrote. I know the age is for Yichud for a man is 9 and a day. I used 13 because, as you noted, if the Jewish child is under bar or bas mitzvah and ther other person is a non-Jew, there is no one who has an obligation. However with a Jewish boy over 9 (and a non-Jewish adult female) we would/should still treat the situation as Yichud for chinuch purposes absent some grounds to be meikel.
I never said that 65 was elderly. I wasn’t basing what I writing on the idea that one of them was incapable of further aveira. I was merely creating an example of a reality where coming to a more serious isur is unlikely and therefore Morei Horaah are more likely to be meikel if they can find grounds to do so. The particular kulah would depend on the particular facts of the particular situation.