'Girl' can actually be used for any female at any age - sometimes as a compliment if someone says you are youthful looking. A girl can also be referred as a female child, a school child, a daughter.
We tend to use 'young woman' or 'young lady' once the female has reached a certain maturity from the age of 14 years old - so as not to make them feel a primary school child but a young teenager. At 14 girls enjoy putting on make-up, buying clothes, dancing, singing, chatting and socialising with their friends.
I can also use the term 'young lady' or 'young woman' if I was cross with a teenage girl - sometimes the mother or teacher will use this.
All the other comments from the others stand too.