Pjiang, Xiaobaitu and all da xia:I-824(follow-to-join) questions

I got my GreenCard in July 2008.
I have a child of 17 years old in China(her dad has her custody).
I was told if I apply I-824 for my child

1. I have to file it within six months of my I-485 approval(that mean I only have less than 4 months).
2. In order for her to remain my dependent, she must be unmarried and under the age of 21. If she is 21 or older and/or married, she is no longer my dependent and cannot apply for a green card based on my approval.

How about I file I-824 AFTER six months of my I-485 approval and my child is unmarried and under the age of 21?
Many thanks for your answers.