Because Sometimes More is More
I’m all for giving people as much autonomy as possible to make this choice, whether it’s through de-medicalization like in Netherlands or UK, or through legislation proper. It’s hard to think that early teenagers have enough understanding of what the surgery entails, and being pressured by your family and/or community at, say 13, seems pretty problematic. As far as I know, in the UK the minimum age for a non-therapeutic labiaplasty is 16, so it makes sense that it would be the same for male circumcision.
I guess I might be the more lenient of the prohibitions, and a soft ban would be OK, although I do see the potential flaw in that kind of thinking.