Old Mojang accounts without COPPA approval that have an email address that isn't in use anymore cannot be migrated. This is due to the fact that you first need to change your email address, but to do that you must first update your account with parental consent. This is impossible however, because the parental consent update page tells you to migrate your account first, which requires you to have an email address that you have access to. This essentially locks an account forever, especially since these accounts are so old that most email providers have already deleted the email with the purchase ID.