This is interesting to me because knowledge of the 'boogeyman' actually goes back to one of my earliest memories when I was 3 years old.
I remember being at a friend's house. An older female was talking. It may have been my friend's mother, but I think it was his older sister. I don't remember if she was talking directly to me and her younger brother. But in any case, the only detail I can be sure about was that she was talking about the 'boogeyman' and that it was something to be afraid of. I don't recall having any nightmares or subsequent encounters. However, the 'boogeyman' stuck with me for many more years as some kind of scary entity that I needed to watch out for.