Usually "baby girl" refers to a female infant younger than about 1 or (at most) 2 years old. "Baby girl" can also be an affection term of address to use when you speak to your own daughter or possibly your girlfriend (though some may find that unacceptable). I think "baby boy" always refers to a male infant younger than about 1 or (at most) 2 years old. An infant aged 2 or 3 is usually referred to as a "toddler", which (as a word) is related to how they typically walk.