He was too busy to notice me standing there.
My 13 year old grandson 'D' was with me for the weekend as he is many weekends since my daughter and son-in-law divorced. He was in the bathroom with the door unlocked so I didn't know he was in there. I just happened to open the door when he was masturbating and must have been close to cumming when I walked in. He kept going until two powerful streams of cum shot out.
He didn't seem to care that I observed this activity or he was just too far gone to stop. He and I smiled at one another and I left him to clean up. I was so horny at seeing his young erect penis that I went into the other bathroom and had the most intense orgasm I have had in many years.
Later we talked and had a wonderful weekend together. He with his private times and me with my own.