One of the keys to looking aged is to accentuate the preexisting lines on a person's face. Have your subject make several different facial expressions to help you identify the lines in their face. Create a light line with a makeup pen on each line so you can identify them later. You might also want to look at pictures of elderly faces to accurately identify their facial line locations. Draw a quick facial diagram with all the major lines that show aging so you can refer to it later as you work.
The most important part of the proper aging effect is to make dark, distinguishable lines in the face of the subject. It is a little harder to do this with people who have dark complexions, so you need to lighten their face with an even foundation. Choose a color that is just a few shades lighter than their current shade and apply it all over their face. Once the foundation settles, start drawing darker lines on the face to show the deep wrinkles.
Highlights are just as important as the dark wrinkles on the subject's face. The highlights give the wrinkles depth and make them more pronounced. Draw highlights on the face next to the wrinkles and lightly blend them in using a makeup sponge. Don't make the highlights so pronounced that they outshine the actual highlights cast by the stage lights, though. The highlights should only be a few shades lighter than the darkened wrinkles, and the actual stage lights will do the rest of the necessary highlighting.
Latex may be the best option for your subject if the audience will get an up-close look. Latex allows you to make an actual textured face that acts like a second skin. Latex texturing is an advanced art that can look either fantastic or fake depending on the skill level of the person applying it. With latex, you also get a wider range of customization of the face. You can add in small scars and dips in the skin at your pleasure, creating an in-depth character.