再回到你的填表问题,你可以写(我英文不好,你勉强看,自己改通顺些就成):
Even though military service obligation is required for a male Chinese citizen (age over 18 years old), It can be delayed if the person is studying fulltime. And the person who being delayed for military service can be called-up if he is not older than 22 years.
So, between 18 years old and 22 years old, I was a fulltime student. So I was not been called-up to take military service.