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多谢8老师 ！Great to know "the mom leaving does not affect the kid's green card validity"! 

A follow-up question:  What if the mom abandons her green card (i.e. does not go back to the US)?  Will that affect her son's green card validity? 

-No. It is irrelevant to the kid if the mother voluntarily abandons her green card.

Does it make a difference if the son is younger than 18 years old (or older than 18 years old) when the mom abandons her green card?

-No difference.