I've hidden the first skin tone layer for now and started another and i'm going to start painting some marks and spots. Now a lot of the detail you paint you probably would even see in the final texture but its about building up layers and variation in the skin tones.
In the picture above you can see I have faintly outlined the details of the face, also in instead of spending ages painting every single spot (that would would take forever). Experiment with making your own brushes, just create a new photoshop file maybe 200 by 200 in size and Paint various spots in it, then like in the picture above make it a brush. you should be able to see the menu and what i've selected.
Now using different skins tones i've darkened and added spots and blemishes here and there, it don't have to be very neat just building up lot of different tones.
A good thing about painting an old persons texture you don't have to be so neat and precise, I find the hardest texture to paint is a attractive female face.
Now we have a base of a texture here but I need to deepen the colour a lot more. To do that i use a map like the one below, this is just a simple layer painted in nice dark tones lighter and darker in different areas, if you want to use the picture below I included the high res version just click on the picture for it.
Now using the picture above as the bottom layer I blend the other skin layers on top of it using soft light or multiply this enables me the easy control the darkness of the texture. Because the skin shader I use does tend to fade out my texture some what so I have to make sure their quite dark.
Now here's the texture blended with the two light skin layers I made.
Marked in red above your see their overlaying the dark texture using soft light and multiply.
Also notice theirs a light skin tone below the dark layer this is if i want the skin lighter I can just tweak the opacity if the dark skin layer.
Your see we still keep lots of the details and marks from the lighter skin layers.
In the picture above I have just darkened the outer areas of the texture using a copy of my dark skin layer and erasing the middle area as you can see marked in red.
Now i'm going to go though all the layers i've used, some have hardly no effect but they all add up to a nice varied skin texture I guess.
Right marked in red is layer 28. (if your a organized person by the way you might want to consider about naming and colour coding all your layers ;-) ) but I cant the bothered. For layer 28 i've painted light colour spots using a simple custom brush I made, then i've used lighten with opacity at 20% so you can just barely see them. On the right i've pasted in a picture of the opacity turned right up so you can see them.
Now a layer of dark spots made again just by using a custom brush. then multiplied.
Next layer lots of random light skin tones then, layer set to lighten at 20% opacity, i've lightened the area on the left of course so you can see then.
Next up a reddish tone adding a bit more variation and colour.
Next up, loads more tiny spots, a custom brush again and layer lightened with a very low opacity so you can just barely see them.
Now a layer of small darker spots layer set to multiply.
Sorry, but its another spot layer.
Next up there's some stubble layers this was made just with a black layer then diffused to make the dots then erased in the areas I did not want them.
I know, lets add some more spots. :-)