I read that Rupert Grunt is the oldest, born in 1988. Daniel Radcliffe was born in 1989 and Emma Watson is the youngest born in 1990. It was part of the promo for the Prisoner of Azkaban.

Studios are notoriously loose with birth dates because they want the public to believe that their stars are age-appropriate in the films. So whatever birth dates the studios feed you, feel free to add or subtract 2-5 years. You see--math does come in handy.