The Covid-19 pandemic is moving into younger populations, health experts said Thursday, with cases skyrocketing among children, teens and young adults. Early in the outbreak, health experts stressed that older adults were most at risk for the virus that has infected more than 4.8 million people in the United States, but new data from the World Health Organization shows that most cases -- by far -- are reported in people ages 25 to 64. The proportion of cases in teens and young adults has gone up six-fold, and in very young children and babies the proportion has increased seven-fold, WHO said. The increase might be explained by broader testing, greater detection of milder cases and shifting demographics of hotspots, but "a rise in risky behavior after easing of public health and social measures" is also to blame, WHO said.