Pretty sure the answer is
Appalachians. (The Appalachian Mountains)
To the Northeast we stand, Ancient natives of the new world,
The Appalachians are in the northeast of North America.
Proud though no longer tall. Worn by age we still watch over the land,
The Appalachians are one of the oldest extant mountain ranges, which is why they're not terribly high compared to newer mountain ranges like the Himalayas.
Still more noble than our cousin hills,
Not sure about this one; I think this may refer to other remnants of the Central Pangean Mountains such as the Scottish Highlands and the Anti-Atlas range, which aren't as well-known as the Appalachians.