Masks had previously been compulsory from the age of 11, but Paris is tightening regulations given the rapid spread of the Omicron variant.
Children aged six and older will have to don a mask on all means of public transport within national borders from Monday, while carriers will not serve any food or drink for three weeks from the same date.
Prime Minister Jean Castex had flagged up the drinks and food ban on transport last Monday.
A spokesperson for the SNCF national railway operator indicated the latest restrictions mean passengers will not be permitted to remove their face masks even to eat or drink, save for cross-border services including Thalys and Eurostar.
Restaurant services will be suspended on high-speed TGV and inter-city services as from Monday until January 23, SNCF said in response to the decree.
Daily Covid-19 cases in France have been hitting one record high after another in recent days, with the authorities announcing 232,200 new infections on Friday, the most since the start of the pandemic.
But the government has so far tried to avoid measures like closing down restaurants and bars, instead encouraging all to become fully vaccinated with booster shots.