So when we got Janelle the shelter told us that she was 5 years old and gave her the birthday of April 21. We later realized that April 21 was the day that she was brought to the shelter and decided that it'd be better if her birthday was June 10, the day we got her.
Every time we tell someone that our cat is 5, including the vet, the reaction is one of shock and disbelief. Everyone always believes that my cat is only 3 or 4 years old due to the condition of her teeth as well as how lively she is and the way she just looks in general.
Cats can live to be late into their teens (my friend's cat is 20 years old!) and once the cat reaches the age of seven it's time to start feeding him or her different cat food and going to the vet more regularly and consider them "geriatrics."
I realize that it doesn't really matter how old she is, because if she's younger than she really is then that just means there's more time for me to spend with her. I love her and I'll take all the time I can with her.