BMI is a pretty straightforward formula. Multiply your weight in pounds (times a constant, I think?) and divide that by the square of your height in inches. Mine is the same for all the sites I've visited.
You may want to double check to make sure you entered your height and weight correctly.
I've never heard of BMI having anything to do with age.
All that being said, however, BMI is extremely limited in the useful information it can provide.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days