After consulting all of my geology and geomorphology books (14) the only thing that even comes close to "topographic age" is topographic development (also called topographic expression). This is taking into account the structural geology, erosional processes, tectonic activity, sedimentology, fluvial processes, etc. within an area in order to interpret the age of the current landscape and the elements within it. This is similar to a geomorphologic analysis of the area.
I think it's how does mountains are formed, by the pressure that is created in the near area of a subduction zone. How the mountains get bigger at the pass of the years, just how Himalayas are increasing their hight.