Whether it's Halloween, another costume occasion or a role in a community theatre production, you can turn yourself into an old lady--whether you start out as a very slightly older lady, a young lady or a guy. All you'll need is a trip to a second-hand-clothing store, tips for make-up and a few other theatrical tricks.
Remember that it's not just what you wear--it's how you wear it. A little work on the "rack" you're hanging those clothes on is in order. Watch old people on the street--they don't move the way younger ones do. If you're going to be a straight-backed old lady, let the work of keeping that posture show. Tense your shoulder-muscles, shorten your steps, reach out for things with care. Keeping shoulder muscles tense affects the way you turn your head too, doesn't it? Put on your shoes and practice walking in them. Many elderly people look down when they walk--falling is a constant danger in the lives of real elderly people. Bring your shoulders forward, let your head go down; when asked to look up, raise your head but not your shoulders (picture a turtle looking up from its shell). Again, shorten your steps and flatten them out--heavy heel-and-toe is for younger bodies. Put a phone book in a tote bag, and try the cane. Add the glasses and a hat. Clumsy? You're getting older by the minute.
Get dressed. Trousers and top or a trousers-suit works fine--better a size big than a good fit. Older bodies don't have the smooth curves that make younger ones look appealing. Ageing means body parts often get stringy or lumpy. Ageing means often "making do" rather than expensive shopping--partly a matter of money and partly a matter of what you see in the mirror. Put a safety pin in a too loose waistband or add a belt that bunches it up. Your new (old) posture will add other slightly less-than-perfect-fit aspects to your wardrobe. Wearing a dress? Put a slip on under it, girlie--nice ladies do. If it shows just a little bit when you go into lookdown posture, you're just fine.
Add layers. Old bodies are more sensitive to temperature changes, especially cold. Add a cardigan sweater or a vest cut sweater--again, off a bit by size, maybe a bit by colour (two shades of not-quite-the-same blue). This doesn't mean colour-clashing--you're dressing like an elderly person, not a bum. When in doubt, a white or beige cardigan does you fine.
Add accessories. Old costume jewellery--if you're on stage, let it be big enough to show, but not big enough to look ridiculous. Glasses--any kind looks "older" if you add one of those bridles that keeps glasses from falling on the floor. Hat--let's get practical. Forget old "millinery" because it comes too close to ridicule, again. Baseball cap, summer bucket hat, straw--you're a practical old lady, keeping the sun off your head. Or a winter hat--down over the ears. Avoid shawls--old ladies wear those only in picture-books.
Whiten your hair, not too evenly or you'll look like George Washington. Most of us don't grey evenly anyway. Long hair needs to be rolled or pinned up. If you're a poor old lady, rubber-band your long, now-greying thatch into a ponytail (haircuts cost money).
If your natural hair is extremely short, grey it or experiment with a wig (but remember wigs are hot).
Try out make-up. Elderly skin is characterised generally by low sheen and pallor. Usually a dusting of foundation powder one shade lighter than your skin-tone will do the trick. Whiten your eyebrows a bit or powder over them. Lipstick should be red or brownish-red rather than pink, or nonexistent.
Always remember that it's going to be easier for people to judge your age by the way you move than by what you wear. Notice how elderly people move. They reach for the handrails you ignore, take the steps slowly, and watch where they put their feet. Develop the moves, and the costume will take care of itself.
Try to stop short of stereotyped images--especially if you're playing to an audience that contains elderly people! They'll spot you for a phoney in an instant.