How should I know how long it takes for the sense of smell to develop? Surely, we're not just born smelling.
Actually, the fact that newborns recognize the smell of the amniotic fluid that surrounded them in their mother's bodies suggests that they can smell before they're born. The nasal cavity separates from the mouth at about nine weeks into gestation, at which point the chemoreceptors for smelling are formed and ready. By the 13 week, the olfactory nerve's connection to the brain is intact.
Smelling does not require air. The fact that infants begin smelling before they're born should help to explain why immediately after birth, a child can locate her mother's nipple. In one experiment, each mother had one of her breasts cleansed after birth, and when the newborn babies were placed between their mothers' breasts, a significant majority of the infants chose the uncleansed breast.
In another study, after only one hour's acquaintance with their new babies, 90% of mothers recognized their baby's scent from that of a stranger's baby.