In prediabetes, blood sugar levels are elevated, but not high enough to be considered Type 2 diabetes.
The study found prediabetes was estimated at 18% among adolescents ages 12 to 18, and 24% among young adults ages 19 to 34.
Analyzing nearly 5,800 individuals included in a national health survey from 2005 to 2016, the study authors found that "the prevalence of prediabetes in male individuals was almost twice that in female individuals" - 22.5% vs. 13.4% in adolescents and 29.1% vs. 18.8% in young adults.
Nine in ten don't know they have it.
Obesity is the driver for the rise, sometimes in children as young as ten.
Study leaders say urgent intervention and education is needed or millions of young Americans could develop heart disease and other conditions in the decades ahead.
Lifestyle changes such as more exercise, healthier eating, and less processed food can head it off.