While it's not unusual for some people to refer to a woman in their early 20s as a "girl," it's increasingly less common, and you risk offending them and others. People will differ on their personal stance of whether they find the word appropriate. Some will find its use sexist while others won't see a problem. But note the number of people who find the term problematic is very high among those under 35 (roughly half in the UK). Considering a woman under 35 is the type of woman you might refer to as a girl, its important to note that you have nearly a 50% chance of offending them.
I think females age 14 - 18, typically, wouldn't find being referred to as a girl as problematic as they would if they were 18+, as they are not legally adults yet in several respects. Additionally, males in this age group are often referred to as boys. You could draw arbitrary boundaries such as the minimum legal driving age (14.5 - 17), the average age of ending puberty (15 - 17), the age of consent (16 - 18), or the minimum allowable voting age (18). Some people might refer to females in the 14-18-year-old age group as young women as @olivia suggests, but I don't find this usage common, and it feels even more formal than referring to them more simply as women. So I'd suggest using either girl or woman for this age group depending on the situation. If it's a formal or professional situation (such as at work), lean towards using "woman."
The test I use is -- would it be strange to refer to a similar-aged male as a boy? Only in situations where referring to a male as a boy feels normal is "girl" appropriate [without prior consent to use the term]. While I sympathize with the fact that there is no commonly used female equivalent to the word "guy", it just isn't worth upsetting someone over calling something they don't want to be called. So I'd avoid the use of the word "girl" in pretty much all situations where the female is over 18.
This answer is similar to @olivia's, but since my age cut-offs are slightly different, and because I disagree with using the word "young woman," I think it's worth a separate answer. Note that many of the more popular answers saying "girl" is acceptable are nearly 3 years old, and language is continuously evolving. 3 years ago using the word "girl" in this way would have been more acceptable. It is less acceptable now, in 2020.