Wikipedia says the OED and US Census dept agree on middle age being 45–65 (which still isn’t enough, if you ask me), and that M-W now say 45–64. Could it be that you have the valuable early print of M-W, edited by Nasty, Brutish & Short?
I’ve started using “child” because people are beginning to take offence at “boy” or “girl”. You can be “girl” if you want. (There was also “lady”, but that seems to have become completely un-PC while I wasn’t looking. Very awkward when I found out.)