A new study has found that the number of young Americans who watch online videos each day has more than doubled in the past four years. The findings were based on a survey of about 1,700 young people aged 8 to 18. Results were released this week by the U.S.-based not-for-profit group Common Sense Media. The group researches youth technology activity and offers guidance for parents. Fifty-six percent of 8- to 12-year-olds taking part in the survey said they watched online videos each day. That rate jumped to 69 percent for 13- to 18-year-olds.