Well my youngest son started at age 8. The only reason I know this is that for some strange reason he thought he was evil for it. He confided in my daughter, who is 2½ years older than him, and then swore her to secrecy.
He spent the next year periodically diving under our dining room table declaring, “I have something to tell you but you’ll hate me”. I spent a few months trying to get it out of him but he wouldn’t tell me. He finally started sobbing about it and it was that point that my daughter told me what was going on. I immediately got down on the floor and talked to him. I told him that masturbating was normal, that everyone did it even his dad, that there was nothing to be ashamed of.
I wish I had spoken to him earlier but I had no idea that boys could start so young. He could have avoided months of feelings of shame. To this day I still have no idea where those thoughts came from. We are not a family that talks about sex in a shameful way. My husband and I are openly affectionate with one another.