There are a number of variables that impact basal metabolic rate, besides height, weight and age:
- the amount of lean tissue in your body
- whether you're well-hydrated
- whether you're well-rested
- current fitness level
- what medications you may be on
- whether your body has experienced starvation in the past
- whether caloric intake right now is sufficient or deficient
- whether you have any health conditions
- gender (hormone levels and differences)
- whether you have any long-term vitamin deficiencies
- past illness (and corresponding treatments)
There's a number of ways to estimate
baseline metabolism, with some more detailed than others. At the end of the day, though... they are only estimates. Taking the average of different methods is probably as reliable as any one might be for you.
A physiologist who takes your medical history into account and delves into your current lifestyle can tell you what method might fit you best. Without that detailed knowledge, it's a best guess.