Showing real data, the 85th and 95th percentiles of Body Mass Index at various ages
This data from the NHANES I study ( 1971-1974) is now out-of-date, because more recent studies, NHANES II and NHANES III have been performed. However, this was the data that was originally suggested to establish "overweight" and "obesity" thresholds, at the 85th and 95th percentiles. (However NHANES II data was claimed to be used when the definitions became official.)
For historical documentation, here are those Body Mass Index percentiles shown as data in a table:
These thresholds were deemed suitable, once upon a time. But later, the World Health Organization (WHO) decided that simpler definitions were needed, at BMI of 25 and 30, for overweight and obesity, respectively. But all such standards are arbitrary.
The chart below shows how the NHANES 1 criteria compare to the more recent CDC charts criteria. In children, they are remarkably close. But for adults, apparently the NHANES 1 values were too high to use as standards, so the CDC and WHO lowered their criteria to 25 and 30.
From this same article, I’ve also plotted a graph of the 15th percentile BMI values for Men and Women. I think this is important, because it represents skinny people, who remain skinny throughout their lives. Notice that their BMI increases with age. In my opinion, this indicates that it is unrealistic to expect adults to maintain the same Body Mass Index values that they had at age 18.