For this tutorial, I will take in account that you have the minimum Gimp/Photoshop experience, that you know how textures work and that you have at least sucefully modified one texture
What are age maps?
Age maps are nothing than greyscale copies of a certain facial texture, when correctly renamed the Oblivion engine will aply these greyscale textures as a layers on top of you characters face texture depending on the Age value that you set; modifying these textures by adding all sorts of details, such as scars, tattoos, etc... allows to you add details to your charaters face without ever needing to mess with the original texture
Folder Structure and File Naming
Texture - Age Map comparison (Gimp)
So, how does this work?
Simple (as long as you have basic Gimp/Photoshop knowledge)
1- Open your face texture in Gimp/Photoshop.
2- Make it greyscale.
2.1- Background color should be grey #424242
3- Add whatever you want to it;tattoos, beards, makeup, etc..
4- Save it following the naming rules stated above.
5- Make a normal map for it
5.1- fill the alpha channel with #121212 grey
Now in game's Facegen or Construction Set just simply change the Age slider and you'll see the age maps being applied.
And that's it, have fun xD