An African tribe called the Ik throws their children out of their homes once they turn 3. They are left to fend for themselves with no help from their parents at all, and to survive, form groups with others their age. These groups only last a few years, and every so often the individual will join a completely different group.
You come upon a ruined building in the back section of a city park (or other out of the way area of the city). The ruins are fairly overgrown. All that is really standing is a doorway and its frame. If you pass through the opened door, you travel to a different world. If the door closes, there may not be a doorway back to get you back.