It’s a superstition that’s brought down in a few (very few) Seforim as that their teeth won’t grow in, which experience tells us Pashtus isn’t true. It’s also a weird European superstition that a baby who sees its reflection before its teeth grow in will die within a year. That one isn’t true either.
According to a book I have (published by the American Foundation for the Blind), babies with typical vision reach 20/80 vision between 6 and 8 months. One of the activities suggested is to position an infant facing a mirror. By 7 to 8 months, the child should smile at his image and may even try to kiss it. I’d love to see that.