That's actually the wrong question to ask. It's not the baby's age that affects whether they can go in a baby carrier, but rather their weight. Baby carriers tend to have a minimum baby weight of 16 pounds. (And of course, there's also a maximum weight.) You may wish to wait until the baby can hold the weight of their head on their neck before putting them in a baby carrier backpack. This will probably exclude the first three months following their birth.