Hola Pomah! Se considera niña hasta los 14 años de edad más o menos. Chica y muchacha son totalmente equivalentes ( una persona joven). La única diferncia quezás es que muchacha es un poco más informal que chica. un saludo.

Niña means a little girl (up tp 15 more or less). Muchacha and chica mean a young women ( let's say from 15 to 30 more or less). It depends on the area they use more muchacha or they use more chica. I think muchacha is more Southamerican and chica is more Spanish. I personally use always chica and never muchacha.

Román, take into account that Spanish is spoken in a lot of countries and the two answers you got were from people from Spain, so I´m going to expand a bit on them. In most countries of the central and northern parts of Latinamerica, "niña" can also be used to refer to a not so young girl (even someone in her twenties). Then in Argentina, "chica" means both little girl and youngish girl, while "niña" is very rarely used. And here in Uruguay, the use is the same as Asier described, except that "muchacha" is the more formal word, and seldom used by young people.