There are several in Skara Brae. If you have the PC map, they will show the locations. If you don't, they are on this website for download. If you come out of the adventurers guild, go south around some corners and one is on your right.
You can use the Youth Potion which can be found in the inspectable building in Scara Brae.
Later your Chronomancer will get the OLAY spell which cures this effect.
Brian the Fist wrote:Of course ZZGO will heal the withered effect too.
In BTIII, the Dreamspell translated over to "Divine Intervention" and I don't remember whether it healed whithering or not. I do remember having to cast OLAY a fair bit...