I think I was about seven. I'll never forget it because I was on the floor of the living room playing with a toy car and my dad called my mother to tell her the news. He must have been really upset about it because at that point they were recently divorced and hated each other's guts to the point where they wouldn't have any contact with each other at all. Yet he called her to tell her that Lennon was dead. I remember my mother saying "He can't be, I just heard him interviewed on the radio yesterday."
I guess I was too young for it to make any impact on me there and then, but the next day at school was surprised to see teachers upset and the headmaster talked about John Lennon in morning assembly. I think from that point onward, both Beatles and Lennon songs were sung at school instead of hymns. LOL! Gotta love modern teachers. Music lessons were invariably the singing of Beatles songs. We did Penny Lane and Hey Jude quite regularly. When we sang Hey Jude in the Christmas Concert that December, all of the parents were in tears. I can't imagine anyone being that upset by the death of a rock star these days. Well not any adults anyway.