How was Andrew Jackson classified as a westerner from the frontier?

Probably because at one point in time that is exactly what he was. He moved to western Tennessee while a young man. Tennessee was definitely on the western frontier when he arrived there.
By the time he ran for President the western frontier had moved farther west and Nashville where he lived, had lost some of its frontier qualities, but was still considered to be in the frontier west by the people in the old part of the eastern US.
Probably because at one point in time that is exactly what he was. He moved to western Tennessee while a young man. Tennessee was definitely on the western frontier when he arrived there.
By the time he ran for President the western frontier had moved farther west and Nashville where he lived, had lost some of its frontier qualities, but was still considered to be in the frontier west by the people in the old part of the eastern US.