According to classical geology models, we are looking at the following general processes in terms of time sequence:
a. deposition: layers of sediments
b. lithification: during burial
c. uplifting: (may partially overlap with b.)
d. stages of meandering: (may partially overlap with c.)
e. entrenching: (may partially overlap with c, and d.)
The above sequence is the history model of the interested area. So, from what stage to what stage is the time interval which you called its "age"?
If you are only interested in the time period for stage e. in this particular area, then you are having a narrow and incomplete view. The answer to the time period of stage e, is in fact, intertwined closely with stage c. and d.
Think about a river which flows on the flood plain of Nebraska (4000 ft high) today. That is at the stage d. The uplifting of the plateau has already happened at least a few million years ago. But, the river is not entrenching at all today. This feature goes against your simply model of your pet landform. Millions of years has passed, yet the river did not cut one inch down.
I know you are not thinking so much. But what I said is only part of the process involved in the background of creating such a landform. With this understanding, then you can start to think about the process of river erosion.
For those who may think that the description of a river in Nebraska makes sense we offer the following comments.
Nebraska was shallow tropical ocean up through the Cretaceous. Giant sea turtles nesting on the beaches of Wyoming, that sort of thing.
There was a general uplifting, the old sea bed is high above the world sea level now.
The uplift of the rocky mountains produced the source of nearly all the sediment that now is thousands of feet deep over the old seabed. Rivers carried it out across the plain and deposited it there, building the sediment layers deeper and deeper.
A river carrying a lot of sediment will, as any actual geologist knows, aggrade the channel. Ancient stream beds in western nebraska are deeply buried under the silt of newer ones.
And for that matter, a number of them flow thru deep valleys, others are shallow and flow in wide braided channels. (which river is juv talking about)?
The simplistic and untrue statement that ANY river flowing across Nebvaska has not cut its channel deeper can be addressed by simply driving across the state on I 80. Hmm! This road follows the Platte River, and gosh, you can see that it is a flat valley nice wide flood plain and un, you can see the sides of the valley to north and south. If not satisfied you can check the white river, the niobrara, check them all.
Its a bit counterintuitive but... in the relatively dry years, a river will cut its channel deeper, it wetter years, it will tend to aggrade the channel.
as Juv says..."With this understanding, then you can start to think about the process of river erosion.[/quote]"