This is a very interesting question and one that is not well studied or documented in medical literature. There are a few small studies which suggest that reflex smiling is replaced with social smiling at about 1 to 3 months of age. The difference between the two is generally accepted to be eye contact between the infant and the person observing the smile.

I'm not sure I can guarantee a "real smile" at any age, given that studies have shown that humans have the ability to manipulate their lower face for deceptive purposes, and a "social smile" is one of the best deceivers.

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