The first "clear" memory I have of masturbating is when I was about 5 and my mother entered my room while I was doing so
I was ashamed but she reacted well, like I was just playing and then left the room, though I know I masturbated since a long time before that (because in fact I remember masturbating before that) and I think I discovered it by myself when I was bored.
I don't know what made me ashamed of masturbating and scared of being scolded, though. Maybe it happened before that she entered the room and scolded me but I forgot?
Still it's a very strong memory, and I remember the feeling as well even though it's not so strong now; more recent ashamed memories I try to forget, but it's not hard on me to remember this one.