Actually, the Black Hills lay claim to the two billion-year-old mark, too, as this geology article from Northern State University notes:

http://www.northern.edu/natsource/EARTH/Geolog1.htm

There are also some good notes on the Black Hills’ two-billion-year-old rock in Deadwood Magazine, in the Wikipedia entry on the Black Hills and from the National Park Service.

In fact, the Wikipedia article suggests that the oldest rock in the Black Hills could have formed as early at 2.8 billion years ago:

http://en.wikipedia.org/wiki/Black_Hills#Geology

So who’s right? Where are the oldest mountains? As the post says, no one knows. When you start dealing with numbers that huge, geologists can’t pin down specific dates. It’s awfully hard to look at a rock and say, “Hm, yes, that was clearly formed on August 7, 2,398,471 BC.” Errors of a few hundred thousand years – or much more – are pretty common.

And then there’s the question of what you mean by age: are you counting how old the rock is, or when the uplift took place? For instance, the oldest rock in the Black Hills may be 2.8 billion years old, but the uplift that pushed that rock to the surface only happened about 300 million years ago. You know, give or take. You can find some more information from the National Park Service here:

http://www.nps.gov/archive/wica/Origin_of_Wind_Cave.htm

But it’s not that simple, either. As recently as 40 million years ago the Black Hills were still shifting around a bit.

While there’s quite a bit published about the geology of the Black Hills, I wasn’t able to find much about the Porcupines. However, the article you linked to indicates that the rock in the Porcupines “metamorphized,” or formed, about two billion years ago, making them about as old as the Black Hills. I wasn’t able to find any information about when the actual uplift took place.

As the post implies, it’s really unlikely that anyone can lay claim to the “oldest mountains” title with any sort of certainty. It makes a nice line on tourist brochures and the like, but scientifically, it’s more like a my-dad-can-beat-up-your-dad kind of competition.