Go read the Disclaimer again. I am not a doctor. This is not medical advice. Seriously.
We can tell that infant sleep is a cultural goal, because parents who back their way into co-sleeping find themselves forced to defend their not-really-a-decision to people, many of whom have no young children of their own, who insist their child will be ruined forever. Parents cannot avoid this issue by keeping their experience to themselves; questions about how the baby is sleeping are one of the first questions asked relatively new parents. Even parents who look quite well rested may be grilled about their baby's sleep habits and unsolicited advice given.
I talked to a lot of parents before becoming one myself. I heard story after story after story of infants who weren't sleeping through the night for a year or more. I heard a couple of stories of infants who slept through the night immediately, one of whom had major medical problems. Yet despite this compelling personal evidence, I read in the AAP guide to raising children that at 4 months most infants are sleeping through the night, giving time frames that indicate they mean 12 hours in a row. If this were truly a developmental stage, or a reasonable goal for human children, I'd have heard some evidence that some babies sleep that much at four months.
They then followed up that comment with a bunch of commentary on how to help that happen if it hasn't already, and that advice included night weaning and moving the baby out of the parents' bedroom if that had not already happened. Neither of these are particularly great ideas before 6 months if parents are interested in maximizing their baby's health and minimizing risk of SIDS.
James McKenna has done a lot of sleep research on mothers and babies sleeping together. Needless to say, he is not an advocate of infants either sleeping alone or sleeping through the night. He is both scientific and articulate in his objections to the idea that infants sleeping alone for long periods of time at night is an objectively necessary or even reasonable goal.