I agree. If you accept their post-birth abortion premise, then why should there be an age-limit on who can be post-aborted. After all, everyone currently living can be considered “post-birth”.
Why an age limit indeed?
Philip K. Dick’s anti-Roe v. Wade short-story “The Pre-Persons” had the dystopian vision of an America in which anyone who couldn’t do simple algebra was subject to “abortion”, precisely as a reductio ad absurdum of this sort of demonic causistry.