Mind Games - S4-E16
Revealing mistake: When Jen and Jack are having coffee at the coffee shop, Jen walks outside and turns to bang on the outside window of the store to get Jack's attention. When she hits the window, you can tell it isn't a real window made of glass. It's obviously some sort of plastic, as it flexes too easily and moves too much to be a real store window. This was probably done to make the window safer for the actors during this scene.
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.