We're not talking about an act of criminal violence, or indeed anything illegal at all. We're talking about a private, off-the-cuff remark, isolated from much surrounding context or knowledge of what was going on in his mind or heart when he made it. For all we know it was just a dumb 19-year-old joke, so there's not even any clear evidence of immorality, apart from by the person who publicly shared a private conversation.
OK, I understand that for a lot of people, it's really important to fixate on this "gotcha" instance, to take the view that the worst possible conceivable motive just has to be the only plausible one, even though there is no possibility of undertaking the kind of forensic or judicial examination that would be required to properly determine such a thing.
But seriously, regardless of what it may or may not reveal about Zuckerberg, fixating on it is just not a healthy or productive way to look at the world.
I invite you to explore the psychology concept of the shadow [1], and consider that when we fixate on other people's flaws and transgressions like this, there's very likely plenty wrong that we're not seeing in ourselves, but that we could greatly benefit from paying attention to.
That's is why I call out this trope whenever I see it on HN. Not because it upsets me that people are saying mean things about Mark Zuckerberg. It's that in focusing on other people's supposed sins, they're likely avoiding looking at what they could pay attention to in themselves, and thus embrace big opportunities for their own personal growth.
Say what you like about Zuckerberg and Facebook's current/recent conduct. There's plenty there, and there are plenty of valid and important discussions to be had about the role of Facebook etc in the modern world.
But this 15-year-old comment trivialises these present-day issues, and is also just super-boring.
By the way, the Oxford dictionary and Wikipedia both define "teenager" as any age ending in "teen", so nobody is wrong to use it in that way. You can apply a different interpretation, but it would only be for rhetorical purposes, not to correct any errant assertions of fact.
[1] https://en.wikipedia.org/wiki/Shadow_(psychology)