Nothing, Some people seem to react to it as its saying they are not mature.

I think women look at it as condesending.

it's condescending

Nothing, unless you use it the wrong way.

It depends on the purpose, generally. It can be considered infantalizing but it can also be sort of 'youth-granting'. As a general rule avoid calling any female over seventeen a girl because such females are, properly speaking, women. . 'Young lady' is probably a better term for girls nearing womanhood than girls. . As regards infantalizing, in the old South it was common to refer to black males as 'boy' in order to establish a sort of dominance. It doesn't work like that anymore and rightly so. . Females are deserving of respect - but are also rightly respectful of others (human being thing, everyone is supposed to do this). If you find you have offended apologize and say you'll try harder in the future - this is especially important for older people who have developed habits that become out of vogue - and then try harder, especially with the offended party. . If you're the offended party, try to understand that people generally have better things to do that go out of their way to piss you off and probably no harm was meant. Even a half-hearted apology is worth accepting if it can bring peace and dignity. Accept it and move on. For the sake of all that is right and good don't make it your mission to tutor the old - there are an awful lot of them and if you wait long enough they'll just keel over dead anyway. Let them be and remember you'll be eventually be saying things that your pissy grandchildren will want to call you on.

Nothing is wrong with it unless used with the intent of insulting someone. Some women like being referred to as girl - even between themselves, even if they're in a retirement home. Others find it demeaning.