Agreed, it can be very frustrating - and is often used for that very purpose, to let the person know they just wasted x amount of time to compose a message no one is going to even read let alone take seriously. But I do not think this is really a good indicator for someone being a child, it’s just intentional insult.

I tend to think they’re either young or generally underexposed to how human interaction works.

There’s a lot of us who are not very great at communications for various reasons. I personally might just lose interest in conversation and can’t get myself to put in the effort to actually contribute value to it and just phase out, and same happens online in a way. As fucking annoying it might be for the person I’m conversing with, it’s not intentional nor meant to insult