Why does the youngest child ask the 4 questions on Passover?

Answer 1

It doesn't. The four "questions" introduce the "Magid" or recital of the Haggadah.
The Magid stage of the seder is preceded by the Sanctification, then the first washing
of the hands, then the eating of the parsely dipped in salty water, and then the breaking
of the middle cake of matzah. These steps are all conducted before the four "questions".

Answer 2

The seder begins with a blessing over wine, then hand-washing, then a blessing over a green vegetable (parsley), then breaking a piece of matzoh. Then come the four questions, close to the beginning of the Seder. The questions are asked early because their answers form the entire story of the Exodus told during the rest of the Seder. It wouldn't make much sense to tell the story and then have the questions asked. Also, since the four questions are traditionally asked by the youngest child in attendance, and Seders can be long and difficult for small children, it's helpful that they're asked early. See also:

More about the Seder

Answer 3

To keep the children engaged. They used to give nuts and candy (treats) to the children to keep them up and asking questions. In some families, everyone says the 4 questions.