This happened when I was about 13. I was at my house, doing the deal, in my room when my cousin walked in. For your information, I usually masturbate by humping some stuffed animals, so she caught me humping. So, she asked what I was doing, I pulled out and she asked if she could help, I said YES!
So she took my cock and started stroking me. It felt so good (I was almost there when she had walked in), so after about a minute, I said I had to cum and she stroked me harder. I came all over her face and breasts. It's so erotic I still think about it.