Villagers are the people that live on the islands. They are the people being controlled by the player, who is supposed to make the villagers' lives easier.
Age is a big factor in the villagers' output. It determines what the villager can and cannot do.
A baby is the child that is still being cared for by the mother. The baby doesn't have its own Details page, as it is only shown in the photo of the mother carrying the baby. Therefore, it is unable to determine name, gender, abilities, preferences and health (a baby cannot get sick apparently) of the baby yet, but his/her age is surely below two years old. The infant cannot do anything yet, so the mom will spend all of her time on caring for the infant.
Children are villagers with the ages 2-14 years old. A child cannot work yet, so he/she will not farm, build, research, heal and breed definitely no matter what the player does. However, it can do certain functions that adults cannot, like picking up mushrooms, which contribute to the food supply, and starting Virtual Villagers 2: The Lost Children, they can also pick up collectibles. When left alone and they're not playing, dancing or answering nature's call, they might be curious about something which could contribute help to Puzzles. Children's speed are always quick, unless they dislike running. Children can get sick.
Teens are villagers with the ages 14-18 years old. They can work such as farming, building, research and healing, but they cannot reproduce yet. If an adult villager is dropped on a teen, the adult will start telling a story while all of the children and teens will start listening. Teens can get sick.
Adults are villagers with the ages 19-50/55 years old (depending on the game). They can work and reproduce, They can run if they like running, but they dislike it, their speed is very slow.