To extend on @Auspice's point a bit, we're also adults - at least the vast majority of us that would play on a MU. We can play children, sure, it's interesting to write from the perspective of a child rather than an adult in certain instances.
When we TS, we're essentially writing porn. And that's fine, when both participants are adults and are presenting as adults. When you throw under age presentations into the mix, especially if you're combining an underage character with an adult? That's just... messy, to put it mildly.