The part that people leave out is that the “mark of the beast” is more than just a simple mark. The Bible talks about people that take the mark worshiping the beast. That indicates to me that there is some type of loyalty oath involved—something where you disavow God and pledge your loyalty to the beast.
An infant is incapable of taking a vow, hence putting a barcode on one would not meet all the requirements laid down by the Bible for it to be the mark of the beast. The mark will have to be something given to adults (or at least people old enough to make an informed decision to reject God).
IMHO, it doesn’t need to by a physical mark or implant. It could be modeled on customer loyalty cards used by so many merchants currently. You take the loyalty oath to the beast, and you are given a loyalty card. Before you can make a purchase, you must swipe the card (number in the hand) or enter the number on a keypad or verbally tell the cashier the number (our memory is in the frontal lobes of the brain, so literally the number would be stored in the forehead).
No, I obviously don’t know for sure this is it, but we should be wary of reading our current biases for a physical mark into the Bible. God may have used the word mark to describe something that not only hasn’t been invented yet but that we’re incapable of imagining.