The Chinese government has clamped down on the amount of time youngsters can spend playing online games, according to the official news agency Xinhua.Under-18s who play for more than three consecutive hours a day will have limits imposed on the amount of points they can score, the agency reported.Online game companies based in China have been given three months to install the so-called anti-addiction software.It is part of an on-going attempt to limit how long people are online.A recent report from the China National Children's Centre said that 13% of under-18s who used the web were addicted to online gaming.