Washington ? Ever watched a teen skulk in the corner of a toddler-packed pediatrician’s waiting room, obviously wishing to be anywhere else?

Adolescents aren’t just big kids, and too many start falling through cracks in the health care system when they pass the stage of preschool shots and summer camp checkups — what a major new report calls missed opportunities to shape the next generation’s well-being.

The period between ages 10 and 19 is unique, bringing more rapid biological changes than perhaps any age other than infancy. Even though most of the nation’s 42 million adolescents seem to be thriving, it is a time of risk-taking and pushing boundaries in ways that can mean immediate consequences: Car crashes, experimenting with alcohol or drugs, teen pregnancy, sexually transmitted disease.

And it’s also an age when many of the habits that determine good health during adulthood are set, or not.

“They are quite literally our future. If we don’t take good care of them, there’s a strong likelihood when they’re running the ship they’re not going to have a good time running the ship,” said Dr. Frank Biro of Cincinnati Children’s Hospital’s long-running adolescent medicine program.

Yet the system of care for tweens and teens is fragmented and poorly designed. Few doctors specialize in adolescents’ complex needs, or provide comprehensive care that earns their trust, concludes a recent probe by the National Research Council and Institute of Medicine. Most at risk are the poor.

The result: The past decade has brought declines in teen pregnancy and smoking but little other overarching progress. Tweens and teens increasingly are overweight; physical activity’s dropping; chronic diseases like asthma and diabetes are on the rise; and injuries, chiefly from car crashes, remain this age’s leading cause of death.

While 20-somethings tend to see primary-care doctors the least, a gradual falloff begins in adolescence.