Don't know of any grey textures available for the ponytail. What I usually do is copy the original hair texture.jpg, open the copy in a paint program and either change it to greyscale or turn the color saturation way down low. Then I might add some very muted yellow or blue coloring to give it a bit more of the silvery look of an elderly person's hair. It's a fairly quick process. Usually works best with blonde textures, however pretty good results with darker ones also.(Make sure you use a COPY of the original to do this)

I might also add that quite a few old people do color their hair and neither of your renders look unusual with dark hair.