Revealing mistake: In the scene where Kim comes home early from the camping trip, Edward is startled and begins stabbing the water bed, thus causing it to spring many leaks. If you actually look at the bed, it isn't jiggling like a water bed should and the water is just spraying out even in places he isn't stabbing.
Revealing mistake: When Edward is leaning in to touch the deceased Inventor's cheek, you can see the Inventor's left eye twitch.
Answer: As the other response says, the number of years is never stated. Physically, I think he is supposed to look about twenty years old. Bill wants him to start a proper business. None of the adults seem to think that he should be in school.