Like trees, most of us increase in girth as we age

 

Source (UK data)

The main strength of the paper is the thorough (longitudinal) analysis of 273,843 BMI observations on 56,632 participants in studies spanning births between 1946–2001 and ages from 2–64 years. No other study has such extensive serial data covering such a wide range of ages and birth years. 

In terms of weaknesses, 

(1) it was not possible to model separate trajectories for overweight and obesity; 

(2) the trajectories were smoothed over age periods in which no sweep took place and thus did not capture local traits, such as a peak during puberty, for some studies; 

(3) we assume our findings are due to changes in adiposity more so than fat-free mass, but this might not always be the case [51,52]; and 

(4) by excluding non-white participants, we were not able to consider the extent to which secular trends in obesity might be driven by the changing ethnic composition of the UK...

The measurement protocols for weight and height were not consistent within and between-studies, which could have introduced bias if, for example, self-reported measurements were systemically under or over-reported. The tendency of people with greater BMIs to under-report weight suggests that our results are conservative

For a man of average stature, he becomes "overweight" at 169 pounds. For the average woman, she becomes "overweight" after she passes 143 pounds.

Those same individuals have to gain 34 and 29 pounds respectively to graduate from "overweight" to "obese".

In the 1946 (immediately after 10 years of economic depression and 6 years of World War, 1958 and 1970 Body Mass Indexes were virtually identical for 10 year-old children.

From age 10 and after, the boys/men gain weight decade-by-decade for the duration of the study.

In the 1946 study, women's BMI remains virtually unchanged until 1960 when those women are approximately 30 years old and then their weight slowly climbs.

The 1958 study shows women's BMI starting to creep upward starting at an age that is eight years sooner than the 1946 cohort.

The 1970 study shows women's weights starting their upward creep even sooner.

The 1991 and 2001 data show both boys and girls with approximately 10% more individuals in the overweight or obese categories. That is 2X the "floor" from the earlier studies.

The tabular data for the 1970 cohort shows 7% of 10 year-old boys, 18% of 20 year-old men, 52% of 30 year-old men and 67% of the 40 year-old men being overweight or obese. It also shows 12% of the 10 year-old girls, 16% of the 20 year-olds, 34% of the 30 year-olds and 49% of the 40 year-old women being overweight or obese.