For many artists, humans are highly challenging subjects to paint. Older humans provide an additional challenge because of their wrinkly skin and specific characteristics found only in older bodies. For example, the eyes of an older face are often slightly sunken in the sockets, the cheekbones might stand out more and the jowls might be sagging or drooping. Painting a realistic senior citizen is something that can only be accomplished by studying a photograph or a live model.
Find a photograph, or a live model. There are advantages and disadvantages to both. While a live model may become tired and lighting conditions (especially daylight) are subject to change with time, a photograph will remain the same no matter where it is placed or when you work from it. However, a photograph will need to be large, in focus and high-resolution in order for you to paint from it. If your photograph is too small or not detailed enough, you won't be able to effectively paint the figure in it.
Set up your canvas on the easel and tape the photograph (if you are working from a photograph) to the easel where it will be visible as you paint. If you are painting from a model, set up your model. Make sure your model is sitting down in a comfortable pose, without arms above the heart. Outline the body of your model on the chair in chalk, so that your model can take breaks and then resume the position. Take a few pictures of your model so that you will have them to work from later. Draw the bare essentials of the face on the canvas.
Draw the outline of the figure, leaving out all but the major details (like eyes, nose, mouth, and if you're drawing the full body, lines of the clothes), on the canvas. Make this painting a portrait/bust -- don't draw the body. This will give you special practice painting the wrinkles and structure of an older face.
Mix the flesh tones in paint. For lighter skin tones, use red, yellow, white, brown and a tiny bit of blue. For darker skin tones, use brown, red, yellow, blue and a tiny bit of white. Use a paintbrush to swirl the appropriate paints together on your palette until you are happy with the results. At this point in the painting, aim for a mid-tone flesh tone (not a highlight or a shadow). Keep in mind that oil paints take a long time to dry (weeks), so any extra paint on your palette should keep for at least a few days. Feel free to mix up a generous amount of paint on the palette, and if you have extra left over at the end of the session, you can save it for the next one.
Paint the first layer on the canvas. Thin the paint a little by dipping the paintbrush in the paint thinner before dipping it in the paint. Paint the first layer of paint quickly and without much attention to detail. Don't worry about painting wrinkles on the face yet.
Build up layers of colour with increasingly thicker layers of paint. Switch to a smaller detail brush to paint the first lines of wrinkles on the face. Darken the paint to make the lines of the wrinkles by adding brown or blue to some of the paint you already used for the skin tone. Paint lines on the face as you see them in the photograph or model, following the trails of the lines as they appear in front of you.
Blend the lines of wrinkles you just painted with a dry paintbrush. Go over the lines with the paintbrush you used to make them, then blend that line, then darken again, building up the wrinkles.
Paint the mouth, eyes and nose. Use a small, rounded paintbrush. To paint the lips, add a little red to the flesh-tone paint. For the nose, paint the shadows around the nose, but avoid painting a straight-on outline. Mix the colour of the whites for the eyes by mixing a small amount of white with a little yellow or grey.
Paint the finer lines on the face with an even smaller paintbrush. Don't worry as much about blending these -- they are too thin to be blended. If you blend them, they will disappear.
Add shadows to the face, especially above the deep-set eyes. Use the darker flesh tone you made for the wrinkles.
Mix paint for the hair colour. To make grey hair, you may either mix white and black, or mix blue, brown and white (for a grey with more depth). Apply the paint for the hair with a medium-sized round paintbrush, then paint darker wisps with a smaller detail brush.
Step back from your painting and examine it. If you're happy with the piece, set it aside and allow it to dry.