Will amazon not let you preorder a rated M game if the account age is under 18

Yes if the account age is 17 you can buy the game if your amazon account age is under 17 they will not allow purchase. "Many retailers (like Target, Future Shop, GameStop, Wal-Mart, Toys R Us, Amazon.com and Best Buy) have a policy of not selling games with this rating to people under 17, at least without parental approval. It is the ESRB equivalent to the MPAA's R rating." (see related link) Parental Approval would be not be possible for online sales