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Have you read my above response carefully?
The child CSPA age = the child age when the priority date (PD) is current - the time of I-130 pending. If the CSPA age is less than 21 when the PD is current, then the child can apply for immigration visa with the parents in China or file I-485 within US if they are in US at that time.
 
Note: I-130 pending time =The I-130 approval date - the I-130 receipt date.
priority date (PD) is current = 排期到了.