SINGAPORE—China has a new rule for the country’s hundreds of millions of young gamers: No online videogames during the school week, and one hour a day on Fridays, weekends and public holidays. China on Monday issued strict new measures aimed at curbing what authorities describe as youth videogame addiction , which they blame for a host of societal ills, including distracting young people from school and family responsibilities. The new regulation, unveiled by the National Press and Publication Administration, will ban minors, defined as those under 18 years of age, from playing online videogames entirely between Monday and Thursday. On…