It's not possible to give a single age for the Interior Plains because they were formed over a long geological timescale by a complex interplay of forces. Here's a breakdown of their history:

* Ancient Foundation: The bedrock beneath the Interior Plains dates back to the Precambrian Era (over 540 million years ago), when the region was part of a vast, ancient landmass.

* Formation of the Plains: The Plains were formed primarily during the Mesozoic Era (around 252 to 66 million years ago) by the accumulation of sediments eroded from the Rocky Mountains. This process of deposition continued throughout the Cenozoic Era (66 million years ago to present).

* Glaciation: During the Pleistocene Epoch (2.6 million to 11,700 years ago), vast ice sheets repeatedly advanced and retreated across the Plains, sculpting the landscape and leaving behind fertile soils.

So, while the bedrock is extremely old, the plains themselves are constantly evolving and their current form is relatively young, shaped by the most recent geological events.