a 18-year-old girl 还是an 18-year-old girl?

我也不太清楚 网上有例句Second time comes Beijing is at the central Opera institute study performance, now thought she still thought is very crazy, a 18 year-old American girl person in Beijing.第二次来北京是在中央戏剧学院学表演,现在想起来她仍然觉得很疯狂,“一个18岁的美国女孩一个人在北京.”但也有Don't find fault with her, she is still just an 18-year-old girl. Every coin has two sides.别挑她的毛病,她还是个18岁的女孩.对她要一分为二嘛. 所有还是请教老师 好了