The phrase "six year old" should be hyphenated when used as an adjective before a noun, as in "She is a six-year-old girl."

No he doesnt. Because then, it wouldnt be a twin.

The weight of a six year old should be in between 30-40lbs.

Yes, I plan on purchasing twin bedding for my 10 year old son this year. He is interested in dinosaurs and trains, so I'm looking for twin bedding with one of those characters.

Yes, A six year old can have a six pack. He has abdominal muscles just like you and me. If he eats well and uses them they may become well defined I've seen a six year old with a six pack.