The people with high amounts of mana in their bodies live far longer than those without.
Wielding the Magic
The majority of these mana-rich people have the capacity to wield low-ranking magics, and as such take on magic-related work of lower caliber. They usually enter professions where magic could be involved like blacksmithing due to the high pay.
The minority, those with higher mana than most usually are scouted by their local Academy for education to become full-fledged mages.
The Long Living Families
Throughout
Taeingarnor, there are families that have lived through the millennia with far less passing generations than most.
Examples
The Faros Family
Archturus
Nabidesh
Reviera
Crejurner
Bower
Varngoven
Savros