回复：多谢您，还要再麻烦一次，

提交I-824的目的是为了冻结年龄不要超过21岁，为什么会有大于21岁的情况，如果大于21岁，就不能用i-824follow to join,不知我的理解对吗？
-Correct. If he is > 21, he is not your dependent. When you file I-824 before his CSPA age turns 21, his CSPA age is frozen, which mean he will have no age-out problem. Thus, he still can apply for immigration visa in China using following-to-join to become a green card holder even if he is > 21 when your PD is current again.. Of course, if your PD becomes current before his CSPA age is < 21, he can just file I-485 if he is in US at that time.

请指正。谢谢您。