Well there are a few reasons why a kidmin would, or wouldn't, be hired. To start with, you often see younger kids acting immature as opposed to the older kids, especially on the internet where they can retain anonymity. As well as being immature some kids are known to be stubborn and remain one-sided when it comes to certain topics (i.e. favoritism). For example: let's say a younger admin (I'd rather not point any fingers but here's a great example) approaches you and asks you just broke that poor man's face in with a toolbox. Before you can even start typing it just becomes bwoink after bwoink, each one with less punctuation than the former. At this point he's already banned you and now you're sitting in your chair, phased by what the fuck just happened.

To elaborate: kids are stubborn. Aside from general stubbornness you may see younger kids being hired as admins for possibly three reasons: they are related to the host, they are friends with the host, or they paid off the host (or the server is actually desperate for admins, but when does that really happen). I mean there certainly are a few of the more mature kids out there but they can still have major fuckups (I mean everyone can but these are often recurring), and would have less experience as an admin (and possibly with the game itself!) than say some young 23 year-old hotshot. But there's also respect, self-control, and just good ol' public decency; Nobody wants to see you abusing your powers, replying with lewd or hateful statements (in Ahelps) and look at your greasy face as the lobby picture (I'm looking at you Hippie Station). Kids are just... well they can't quite suffice for such a job. (I mean it's like managing a "real job" but with far less responsibilities).

TL;DR: Kids are generally not socially developed enough to handle administrating things such as servers.