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."

Hi Ana, The problem is that English doesn’t have a specific word for adolescent females, females between puberty and legal adulthood at age 18. Speakers get around this with the common term “teenage girl” or more formal terms like “young woman” or (most formally, from parents, older relatives, and teachers) “young lady”. What *is* odd to English speakers is simply “woman” for someone under 18. Speakers are much more likely to do the reverse: use “girl” for females over age 18. Then they need to say “young girl” for a female before puberty. It’s confusing! But no, “woman” is odd for someone under age 18.

If she is older than a child, then she would be considered a young woman or a woman.