Uncle Lloyd's Tips and Tricks:
I've had a lot of practice playing aged characters, so here's what I've learned:
Grey hair is the easiest way to signify old age. There are two traditional routes:
Downsides: It falls out when you move, giving you sugar-frosted shoulders. As you sweat, powder will drip down your face, and will make your hair cake up. Along the same lines, you'll have to shower for some time to get it out.
Upsides: Readily available at any drugstore, and for cheap.
To apply: sprinkle liberally, then shake your head gently to get it to blend in. Repeat as needed. Have someone check the back of your head when you're done.
This doesn't have to be the really serious dye that takes days to get out. The stuff I use I find in drugstores every October for a couple bucks a can.
Downsides: Hard to find. Every October I buy every can of white I can find, so that I have enough to last me out the year. You might have to look in costume shops and the like.
Upsides: It washes out easily (one shower will do it), and you don't leave clouds of powder behind when you turn your head.
To apply: It helps to have someone else help, since it's hard to spray the back of your head. When spraying your hairline, cover your skin or costume with a paper towel and have someone else spray it on. This stuff doesn't wash off skin that easily, and you don't want white patches on your face or clothing.
Don't just focus on your hair, though -- white eyebrows are also critical. Dab a little white pancake makeup (from the Quest makeup stock) on your eyebrows and you look much more convincing. If you have facial hair, get that too.
Wrinkles are crucial to looking older; grey hair without them gives you that middle-aged "Steve Martin" look. You'll need an eyeliner pencil; I prefer brown-black but your mileage may vary. (For the record, a guy in the makeup aisle scrutinizing the eyeliner pencils will draw stares from women.)
To find the wrinkles to draw, stand in front of a mirror. Now raise your eyebrows as if you're really surprised. See the lines that form on your forehead? Now relax your face, and draw lines over the wrinkles. Then smudge them down with your fingertip until they look less like pencil lines and more like wrinkles. Next, squint your eyes, and draw in your crow's feet and lines below your eyes. Next, smile widely and draw your laugh lines.
Acting old is more than just makeup.
Walk from the knees, rather than the hips or the chest. That is, imagine there are strings attached to your knees being pulled forward, forcing you to move forward. Lead with the knees (the inverse is true when playing a child, for the record. Walking from the head makes you appear younger). This works because it lowers your center of gravity.
Walk slowly. Act as if your arms are a little heavy. Squint. Make your voice gravely or creaky.
To look middle-aged, do the above, but less so. Draw the wrinkles shorter and lighter, for instance. Make your hair only slightly white, or just grey the temples. Leave your eyebrows and beard dark. Leave your voice and posture normal.