Ageing can be done with latex, greaspaints or alcohol activated paints. For latex the skin is pulled taught while latex is stippled on and then dried and powdered between each layer before the skin is released to reveal wrinkles.
I used greaspaints to do some theatrical ageing on a face while at college and currently in my theatre job in Gorey little theatre.
Here is the before and after using latex to age the skin
Here you can also see aged hands using alcohol activated paints an latex
Theatrical ageing using greasepaints is shown below