I totally figured out where there are so few kid avatars in medieval/fantasy RP sims. Stay with me on this.
Read 99% of the backstories of RPers. Almost all of them are orphans. Their parents were killed by orcs, their parents were kidnapped and tortured to death by the enemy army, their parents were killed who-knows-how many ways and now they’re out for revenge. It leads one to the natural conclusion that having children must be a death sentence. Like falling in love with a Cartwright (WARNING! TVTropes Link!). So no wonder so many RPers are practicing whatever form of birth control they are!
It all makes sense now!