A leading cause of certain chronic conditions — obesity — surges in Americans’ 20s, more than doubling between the ages of 18 and 30. It peaks at about age 50 and remains prevalent among Americans in their 60s and early 70s before tapering off.

The Gallup-Healthways Well-Being Index determines obesity on the basis of respondents’ self-reported height and weight, using traditional Body Mass Index (BMI) scoring. Across 2009 and 2010, an average 27% of Americans were obese, defined as having a BMI of 30 or higher.

Whether the decline in obesity later in life mostly reflects the higher mortality rate of obese people as they age or significant weight loss among seniors is not clear.

Younger Americans must make conscious lifestyle decisions to reduce their weight or they are setting themselves up or increasing rates of weight and diet-related chronic medical conditions later in their lives.