小白求助 结构体定义变量的问题

请教各位大佬 结构体定义变量的 啥时候定义结构体元素TEAC t1;啥时候 定义结构体的指针TEAC* t1;
typedef struct Teacher
{
    int age;
}TEAC;

void main()
{
    /*TEAC t1;
    t1.age = 22;
    createTeacher(t1);
    printf("%d\n", t1.age);*/

    /*TEAC t1;
    int num = 3;
    t1.age = 33;
    createTeacher(&t1, num);
    printf("%d\n", t1.age);*/

    TEAC* t1 = NULL;
    int num = 3;
    /*t1->age = 33;*/
    /*t1 = createTeacherC(num);*/
    createTeacherC(&t1, num);
    printf("%d\n", t1->age);
    system("pause");
}