The age of the player is not determined during the world cup. It is determined on 1st of September before the world cup. E.g. For ICC U19 World Cup 2014 the age of the players was determined based on 1st September 2013. That means any player must be under 19 on 1st Sept. 2013 to be qualified. You can see some players like Vijay Zol, Sanju Samson were over 19 during U19 WC 2014. But they were under 19 on 1st Sept. 2013.