"As the old saying goes, you're as old as your arteries are," said Dr. Geetha Raghuveer of Children's Hospital in Kansas City, who led one of the studies. "This is a wake-up call."
The studies were reported Tuesday at an American Heart Association conference.
About a third of American children are overweight and one-fifth are obese. Many parents think that "baby fat" will melt away as kids get older. But research increasingly shows that fat kids become fat adults, with higher risks for many health problems.
More alarming studies are cited, describing the damaging effects of being overweight on children's hearts.
The study involved the first 150 children participating in a larger community-based study.
Earlier research he helped conduct found more rigid arteries in such children - a possible sign of plaque deposits starting to form.
"Even at this young age of 10, you can have children who have got arterial stiffness who are comparable to 30- and 40-year-olds," he said.
Dr. Michael Schloss, a New York University heart disease prevention specialist, said the evidence shows obesity is more than a cosmetic issue for children.
"If you've seen what's on the menu for most school lunches, these findings are no surprise," he said. "The time has come to seriously deal with the issue of childhood obesity and physical inactivity on a governmental and parental level."