At what age one stops calling a person 孩子？
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