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ANSWER: At the time he is eligible to immigrate to the US, if his age, minus the amount of time it took to process the I-130, is less than 21 years old, then he can still come.
But if, at the time he is eligible to immigrate to the US, his age minus the processing time of the I-130, is 21 years old or older, then he is not eligible to immigrate at that time. His category will change and the wait time for him immigrate will change accordingly.
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