There are regional and social variations. In the U.S., "child," "boy," and "girl" is often used for children under roughly age 12 (pre-puberty), "teenager," "teenage boy," and "teenage girl" for ages 12-19 (post-puberty), "young woman" and "young man" for ages roughly 16-25, and "woman" and "man" for age 25 and up.
In the U.S., calling an adult woman a "girl" is often considered offensive because it implies that women are not equal to men because women are "child-like." There are certain social situations where a woman can call her friends "the girls" as in "I'm going to dinner with the girls (my friends)."
Oxford Learner's Dictionaries:
[countable] (usually in compounds) (old-fashioned, offensive) a female worker
an office girl
Sometimes, an adult will praise a child saying "you are such a fine young woman or young man." Similarly, an adult will sometimes insult a teenager saying "you are just a child."