Okay, so first, I completely disagree with your conclusion; there's absolutely things from my childhood that I wouldn't want on the internet even today as a grown adult. Not because I imagine they're uniquely embarrassing and would somehow damage my "current persona", but simply because they're a part of
my childhood and
my person, and it would be an encroachment of my privacy if they were publicly available. It's one thing to show your relatives and neighbors pictures of your children and their drawings; it's another for them to be irreversibly published on the most public platform in existence.
Second, which I'd argue is in fact more important, do you realize how damaging it can be at age 14 to experience this sort of betrayal of trust and elimination of privacy? It's as if the bully in your class got to read your diary aloud - times 100x. The content doesn't really matter, the transgression of boundaries does. And of course you might laugh about it once you gain the distance from your past self and its worries and pains, but it doesn't make the event any less exposing and possibly traumatizing. Adolescence is a brittle time in the development of a personality, and the kind of experiences one has aren't any less impactful because they're "just kids".