I am certainly no expert on this subject, however, if you visit a shelter and view some of the oldest cats there, you might get an idea. I adopted an older cat last year, (Max) who is at least 10 years old. I remember looking at him in the shelter's cage, he looked so dignified, and yet disappointed because of his former owner's demise. However, Max has become one of my favorite lap-cats. He always sits and waits for me to sit down to watch TV. Right now we have a 13-year old cat at our shelter now, (Butterscotch), who looks to be older, just like Max did. However Butterscotch is a most affectionate cat, and loves to be held and cuddled. I hope that someone will take him for their own pet. Cats generally don't show signs of aging until they are about 10 years old. Also, look at their teeth, as is stated before. One of the most favorable facets about adopting an older cat is that they, like older people, make good house-only pets, and are generally affectionate, and do not get into trouble. Also, their coat begins to gray and become more ruffeled than a younger cat. Sometimes I wonder if their coat loosing much of it's "luster" isn't due to a poor diet. When I first brought Max home with me, his coat didn't look too good, however, now he looks much better. His coat is more orange and has regained much of it's color.
When they are kittens it is much easier -- you can tell by the teeth that have come in. You can also tell if a cat is an adult because they get one more adult tooth at that time -- in the way back. But other than that -- it's pretty much a guess. If a cat is a stray and their teeth are pearly white, they are most likely a young adult.