The above calculation does not take into consideration lean body mass. The more lean muscle your body has, the faster your metabolism or the more calories your body will need. On the other end of the spectrum, the less lean body mass/muscle you have the less calories your body needs.
Just one of many reasons a prescribed resistance training program is essential to build lean muscle.(Read Muscling Up for other benefits). Around age 25 our metabolism starts to decline unless we are consistently involved in resistance training.