Thanks to question_asker for improvement in the answer
Pretty sure the answer is
Adirondacks. (The Adirondack Mountains)
To the Northeast we stand, Ancient natives of the new world,
The Adirondacks are in the northeast of North America.
Proud though no longer tall. Worn by age we still watch over the land,
The Adirondacks 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. The Scottish Highlands aren't as high and the Adirondacks and the Anti-Atlas Mountains aren't as well-known as the Adirondacks, at least in Anglophone culture.