In our family we have phrased things that when you hit 10 you begin the journey to adulthood, and that lasts 10-14, then you are ready to take on the responsibilities of a young adult. For us it is about respect, about treating them as people in charge of their lives and bodies. We don't want them to feel 'parented' and oppressed through their teen years (13-19) and then suddenly be expected to be mature adults.
(our eldest will be 10 soon.) we think treating them as respected young adults first will help them act as such.
As for the ear piercing, I'd want to chat with him about why, and if he's ready to care for it and manage it, whatever age suits. I can say all that with children who have no interest in piercings
but we really try to parent from a place of respecting their choices, and if they want such a thing, it's not really harmful so go for it.
When my kids were younger I had big ideas about not allowing stuff I felt was too mature (hair dye, etc) but as they age I find I need to re-think my outlook on all this stuff, so I don't limit them in ways that are disrespectful. If they want it, they want it.