If you are a U.S. permanent resident who filed an I-130 visa petition to start the green-card application process for your spouse and your spouse’s children when they were under 21, but the children turn 21 before your spouse is able to get the visa or green card, the children have a problem. With your spouse still waiting for an immigrant visa or green card, your children who turn 21 instantly lose the ability to get their visa or green card as "derivative" children of the immigrating parent in category "F2A."
They haven’t lost visa eligibility altogether. The unmarried son or daughter (age 21 or over) of a U.S. lawful permanent resident (green card holder) is eligible for a green card; but in category "F2B," which tends to have a longer wait than category F2A (which is only for spouses and their derivative children who are unmarried and not yet 21). (For details, also see When Children Can Immigrate to the U.S. With Their Parents, as "Derivatives.")
After the child turns 21, what should you do to preserve his or her ability to apply for a U.S. green card? One possibility is to request a “recaptured” Priority Date along with filing a new visa petition, as described in this article.
First, you’ll need to deal with the fact that the child who recently turned 21 is no longer included in the immigrating parent’s visa-petition approval.
One possibility in this situation is for the immigrating parent to, upon reaching the U.S. in lawful permanent resident status, file a new visa petition (Form I-130) for the son or daughter. This will definitely require years of waiting, however, and won’t come with the possibility of “recapturing” the Priority Date described below.
There's another possibility, which might work if you (the U.S. green card holder) are also either the biological or legal parent or qualify under U.S. immigration law as a “stepparent” (because you married your foreign-born spouse while the children were under 18). In any of these cases, you could file the new I-130 visa petition. Because of the way the recapture law works, it’s better if you can do this filing (rather than having the immigrant parent file a new petition).
Because you filed the original I-130 petition that the son or daughter was hoping to immigrate through (the one for your spouse), you can ask USCIS not to put your son or daughter at the bottom of the F2B waiting list, but to grant the same Priority Date that your spouse (and your son or daughter) had after their initial I-130 approval. (A Priority Date literally shows when USCIS received the person's visa petition, and becomes like a number showing the person's place in line for a visa.)
This way, your son or daughter will get credit for all the time spent waiting before turning 21. USCIS will pretend like you filed a separate I-130 visa petition for the over-21-year-old at the same time that the visa petition for your spouse was submitted, perhaps years ago.
This is called “recapturing” a Priority Date. Below is a sample letter showing how to ask for a recapture. You also need to include a copy of the original I-130 (if you have it) and the INS or USCIS approval notice showing your spouse’s old Priority Date.
Approval of recaptured dates is supposed to be automatic. Unfortunately, the USCIS Service Centers aren’t used to this procedure and may pay no attention to your request—even if you write the most compelling letter and include complete documentation. You might get an approval notice showing a new Priority Date rather than your family’s old one. Your only recourse in such a situation would be to write many letters trying to get USCIS to correct the date (or to hire a lawyer).