Summarize:

Depending on what age your cat is, its coat may be more or less thick. Cats shouldn't go bald or lose their hair, but by determining just how full their coat is you can narrow down their age. The coat of an older cat may be more thin than that of a younger one. Seasonal variations can occur. Summer coats are thinner than winter coats. If your cat is losing fur, visit with your veterinarian. There are some subtle differences in the texture of your cat's coat at different times in its life. By looking for these differences, you may be able to get a good sense of how old your cat is. Young cats will have smooth and full coats of hair. Older cats will likely have coarser hair. Some patches of gray hair may appear in older cats. As your cat ages, levels of activity will change. These changes to how active your cat is can lead to changes in its body shape. Judging the shape of your cat's body may give you a clue as to how old it is. Young cats tend towards being lean and muscular, due to their high levels of activity. Middle age cats may be more filled out and round. Older cats will likely have pronounced shoulder bones and loose skin.
Check your cat's coat for how thick it is. Feel the texture of your cat's fur. Notice your cat's body type.