It's difficult to definitively say which mountain range is the absolute oldest, as geological processes constantly reshape the Earth's surface. However, the Barberton Greenstone Belt in South Africa is considered to be one of the oldest known mountain ranges, with rocks dating back over 3.6 billion years.

Here's why it's hard to say for certain:

* Erosion and uplift: Mountains are constantly eroded by wind and water. Over millions of years, even the tallest mountains can be worn down to nothing. Additionally, tectonic plates can uplift and create new mountain ranges. This constant cycle of creation and destruction makes it hard to pinpoint the oldest.

* Rock formations: While we can date rocks, it's not always clear whether a particular rock formation is the original part of a mountain range or a later addition.

While the Barberton Greenstone Belt is a strong contender, other ancient mountain ranges are also considered very old:

* The Canadian Shield: Parts of the Canadian Shield are around 3.5 billion years old, and it was likely a mountain range at one point.

* The Baltic Shield: This ancient shield, covering parts of Scandinavia and Russia, also dates back over 3 billion years.

Ultimately, the "oldest" mountain range title is a bit tricky to assign, but the Barberton Greenstone Belt is a fantastic example of the Earth's incredibly ancient geological history.