Q: When someone's green card has been expired for 15 years can they renew?

A: The LPR status for your parents was likely considered abandoned years ago. You will need to re petition for them if they would like to be LPR's. They may also apply for visitor visas. However visitor visas may be difficult for them to obtain because of the evidence of immigrant intent. For visitor visas they will need a lot of evidence to indicate that they intend to return to Iran upon the expiration of their visas.

A: I agree, but you will need to file to naturalize and become a U. S. Citizen before you can petition your parents, so that they can become lawful permanent residents, again. They may apply for another non-immigrant or immigrant visa, but they must formally give up their lawful permanent resident status, first. At this point, because they remained outside the U. S. for so long, it is unlikely that they will prove that they intended to permanently reside in the U.S. at this point. However, more information is needed.

There may be other issues, since your parents are from a nation whose citizens go through additional vetting, among other concerns due to their past status as lawful permanent residents. As a result, I strongly recommend an appointment with a competent and experienced immigration attorney as soon as possible to avoid further complications. The above is general information, not legal advice, and does not create an attorney client relationship.