They have calculators with age as a factor. Really, age just adds a wider range of normal, on both ends.
Body shape isn't much of a factor, expect in cases where fat is stored mostly around the midsection. That actually makes a lower threshold for normal BMI.
BMI is an indicator, not a tell all. It's not supposed to be taken so seriously, just as a general idea.
my waist is the widest part q.q