Most are starting before they are even a year old — and by age 3, they're using the devices all by themselves, the team reports in the journal Pediatrics.

The survey was done in a single urban pediatric clinic in Philadelphia, and the researchers note that the findings do not necessarily extend to the whole country.

But they paint a troubling picture of populations of low-income and minority babies, and toddlers being kept quiet with televisions or tablet devices streaming cartoons.

"At age 4, one-half of the children had their own television and nearly three-fourths their own mobile device. The most popular device was a tablet, owned by two-thirds of 4-year-olds," Dr. Hilda Kabali of Einstein Medical Center in Philadelphia, and her colleagues, wrote in their report.