When I was in my teens I was able to suck myself off. This was due to a combination of having a rather large member, flexible, and a keen interest in doing so. . It was not the greatest head I have ever had nor the easiest. For those wondering, I would come in my mouth sometimes. Why not right? There were a few times when I was fooling around with a girl before sex and we were giving each other oral, I would say hey check this out. I would go down on myself. It was a huge turn on for the girl I was with. She couldn't believe it. She told one of her friends and I was OK with that. Her friend wanted to see me do it. I said I am not some freak show circus act. I said only if you strip naked and pleasure yourself. My girldriend and her friend obliged. She didn't believe I could do it and I bet her friend a 3some I could. I did and then we all had a 3some.
When I got older and less flexible, I could not do it anymore. I still tried and hurt my back a few times. I am now content that I used to be able to and don't try anymore.