“Girl” describes an old-fashioned description of womanhood.  Kincaid drew on her experiences growing up in Antigua for the setting and themes of "Girl," as she has done for the rest of her fiction … from the time Kincaid was born in 1949 until she left in 1966. (enotes, historical context).  This advice would not likely be useful to girls today:

Wash the white clothes on Monday and put them on the stone heap; wash the color clothes on Tuesday and put them on the clothesline to dry; don’t walk barehead in the hot sun...

Girls today don’t really need lessons in sewing or washing clothes on a clothesline.  Sweeping is still a useful skill, both for boys and girls.  Her advice on not being a slut, and causing an abortion, are a little outdated.

However, there is some advice that I think would be useful still.  For example, “always eat your food in such a way that it won’t turn someone else’s stomach” is still good advice.  Also, “you mustn’t speak to wharf-rat boys, not even to give directions” can still be important, although we all have the boys we need to stay away from.