William,
You appear to have questions covering more than one game. The GC only exists in VV1. You can go
Here to find the answer to that question.
It can be hard to tell if a villager is a teen vs adult, but that is easily solved by clicking on the villager in question and looking at their detail screen.
Villagers teaching kids applies only to VV2. Any adept parent dropped on a child will start "teaching" all the children in the area. The adult gains parenting skill for this.
In VV2 & VV3, completing collectibles will increase your population limit. There are no collectibles in VV1. In all three games, if you have 1 less than your population limit (with or without collectibles) and a female has twins or triplets, you will exceed your population limit by either 1 or 2. Having twins and triplets is strictly a matter of luck. There is no way (without cheating) that you can control this.
In all three games, any villager can gain healing skills by healing another sick villager. This is even true of children.
In VV1 & VV2, they can also gain healing skill by studying herbs. In VV3, they can only become doctors if you have the medical lab.