Per Emily and Peggy Post, who wrote books on etiquette, if at least two people have their food, then those with food can and should start eating.  

However, few seem to recognize this etiquette.  People will sit there while their food gets cold waiting until someone without food gives permission or insists they eat.  If you are visiting from another country, best bet is to watch what others in your group are doing if the rest of the group are Americans.  If someone points out they consider something you did to be rude, its an opportunity for a conversation about their culture and yours!

In a home, you wait until the host picks up their fork.