When I was 13 I hid in my sisters (16) closet as an intended prank. It was one of those with the slats so I could peek. I was about to jump out when she started undressing. Soon she was completely naked and took a box out from under her bed. She pulled out a red dildo and spent the next 10 minutes masturbating with it.

I stayed completely still and the whole thing until she climaxed. Luckily for me she didn't check her closet and went to the bathroom. Then I snuck out relieved but aroused.