At what age one stops calling a person 孩子？ I've seen a text where 20-year-old woman was called 女孩子, is it usual in China?

The text I'm referring to:


  我大哥认识一个女孩子，她的名字叫李明英。李小姐今年二十岁，是大学生
Why do you think these sentences are related? I think they are 2 different, unrelated sentences.


我大哥认识一个男孩子，他个名字叫李明英。
My big brother knows a boy whose name is Li Mingying
李小姐今年二十岁，是大学生。
Miss Li will be 20 years old this year, she is a student at university.


小姐 = Miss
男孩子 = boy