Hmmm, that's worrying. I played with some numbers for a 5'6" male, and got this:

99 lbs yields "Your BMI is way too low to be living"
100lbs yields 74 years
150lbs yields 76 years
200lbs yields 73 years
250lbs yields 69 years
300lbs yields 69 years
500lbs yields 69 years
999lbs yields 69 years

It looks to me like they are pulling data from a table, and the table maxes out under 250lbs?