Inspect the cat's teeth to find out if it has its baby teeth or adult teeth. If it has its permanent teeth and they are white, the cat is about 16 weeks. If its back teeth have yellowish tarnish, the cat might be 1 to 2 years old. If all the teeth have tartar increase, the cat might be 3 to 5 years old. More wear on the teeth are generally seen at 5 to 10 years old. By 10 to 15 years, the cat might lack some teeth and have an excessive amount of tartar buildup.