BMI = Body weight (lbs.) / Height(2) (inches) x 703
According to many government agencies like the CDC a healthy weight for someone in terms of BMI is about 18.5 to 24.9 - this means if you're over 25 your over weight. If your BMI is >30 your considered obese etc.
If lean mass and body fat are not calculated into the health equation then there is NO WAY it can be accurate, especially for individuals who carry more muscle. There is an abundance of research that shows someone with a BMI >25 is not necessarily less healthy than someone with a BMI between the 18.5 - 24.9 range.
How do you determine then if the weight that you currently see when you step on your bathroom scale is accurate? You don't - you don't make your bathroom scale the be all end all number, especially since it can't differentiate between the ratio of muscle and fat. Maybe you start to use it as part of your personal health index and add a waist-to-hip ratio (a waist/hip measurement) and a %body fat score into the mix. Start paying more attention to ratio of muscle to fat that your carrying on that body of yours. There are many ways to determine %body fat (Skinfolds, BIA, TOBEC, DEXA, Hydrostatic weighing, etc.) but the least expensive method is skinfold calipers. If you're unable to use any of those methods then take a simple measurement around your waist. This does not determine body fat but it can tell you a great deal. Men should not exceed 40 inches and women 35 inches in regard to a waist measurement. Are you starting to get the picture?
Bottom line - it's not about a number - one number like BMI alone or what your bathroom scale tells you should not define who you are. Start paying more attention to the amount of muscle tissue your body has...you'll be much better of as you age!
Here is a good book to read if your looking to see from a research standpoint why BMI is not as accurate as many believe - The Obesity Myth by Paul Campos.