User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-7418318-20150825070224

So Mavis somehow got the immortality curse rubbed off on herself (directly or indirectly), due to being taught by Zeref. And of the other three who Zeref taught, one was killed and the other two lives far longer than the average, or even maximum length of life of a Human.

Could it be that the whole quartet was affected by Zeref's curse, though only partially and Mavis didn't actually contract it by casting an incomplete law, but accidentally turned it on full force?

That would explain why both Precht and Walrod could get so old, but still stay mortal. And it would also explain how Mavis could get the same curse as Zeref, despite doing nothing that was even remotely alike to the reason why Zeref was originally cursed.