I think part of the problem is this binary label of children | not-children, which then gives rise to infantilizing treatment.
The post you're responding to was splitting the spectrum into at least three parts, children | teenagers | adults, while you're arguing for children | adults - or at least that's how it looks. But I think you're on the wrong track there. Something that is closer to modelling a spectrum is better than a binary distinction, and is less likely to give rise to asinine laws / enforcement, like prosecuting teenagers for self-production of child porn.
There's a lot of room in the justice system for discretion at various levels. The law isn't absolute like that in practice.
I'm more complaining about people - allegedly human - making black and white distinctions. Actually, what I'm more upset about is people working from this self-serving system of "logic":
[general problem] -> [label]+ -> [general action]
For example, saying that casual sharing of media is copyright violation, which is theft, theft is bad and should be punished harshly, thus casual sharing of media should be punished harshly.
It bottle-necks the ambiguity of the real world through a single specific label - ideally one stuck at one end of the spectrum - and thereafter applying a consequence from that label. It's a corrupt mode of thought to my mind.
Regulatory policy tends not to involve "vague undefined gradients", but the trend of replacing the common law process, which evaluates each case on its own particulars, with static, top-down rules and regulations, is itself a bit of a recent novelty, and really hasn't worked out well.
The post you're responding to was splitting the spectrum into at least three parts, children | teenagers | adults, while you're arguing for children | adults - or at least that's how it looks. But I think you're on the wrong track there. Something that is closer to modelling a spectrum is better than a binary distinction, and is less likely to give rise to asinine laws / enforcement, like prosecuting teenagers for self-production of child porn.