The age limit for preference family members - such as children of brothers and sisters - is 21 years old. However, they may still be able to immigrate with their parents if they are over 21 under the Child Status Protection Act - (CSPA) if the case took many years to process.
For example if the child was 18 years old when the case was filed but the case took 5 years to process before you got the approval notice by which time the child is now 23 years old, the law permits you to deduct 5 years from the child's age at the time a visa becomes available. The recalculated age at the time of the interview will be 18 years and the child can immigrate with the parents.
This law is good if the child is younger and the case takes man years to process. The law will not help you if the child is almost 21 years and the case is processed and approved within a few months. In this case, if a visa becomes available in say 3 years when the child is 24, you can only deduct the few months and the new recalculated age will still be over 21.
If you would like a telephone consultation, you may contact me at (410) 992-6602.