修改意见:
What events make a person an adult? Different people have different answers. Some people think that getting married is the symbol of becoming an adult. Others, however, may believe that graduating from the high school will signify that their child become an adult. As far as I am concerned, I totally agree with the latter opinion that graduating from high school makes a person an adult. The reasons are presented as follows.(模板句，还是比较流利的）
...He also can choose the university which he likes by himself and never be controlled by his parents any more.
...that a person has already owned(had) the ability to live by himself. For instance, he can(could) cook meals and wash clothes for himself without the help of the family. ...
What’s more, after a person graduates from the high school, he has studied enough knowledge and skills to get a job in society and make(earn) his own money.

呵呵，开始能把模板套用得这么流利还是不错滴。
写到后来还是自己总结一套吧，其实写得还是不错的！
有一点关于ETS癖好的建议：如果想到了用can,就要考虑改成could,与其委婉些，留点余地，除非能肯定是不争的事实。could,would在T，G考试中用得是比较多的。
呵呵，加油哦！