Hi,
There is no direct Approcah to this requirement you need to use
USEREXIT_SAVE_DOCUMENT_PREPARE
and bild the logic that if the order is more than 3 0r 4 months old than the actual PO date the order need to be blocked with Rejection Reason
or you can use sales order release method based on te status profile
every order will be Blocked with reason maintained in schedule line category and released by Authorized person