A number of discounts options are available when using the RATP subway or bus system. Some of the passes available for the bus and the subway are also extended to cover transportation on the railroad system. If you are residing just outside of Paris and need a combination of subway or bus and train, this might be an option worth looking into.
The Paris Visite is a pass that allows you to travel first class and as much as you want on the public transportation system, for a determined period of time. The Mobilis is a day pass intended for people residing in the surroundings of Paris who need to travel on different transportation modes. More information on tickets is available from the RATP.
As well, based on your age, a number of other discounts may apply. Children under the age of 4 can travel free of charge. Children aged 5 to 10 benefit from a 50% discount. High school students under the age of 21 or university students under the age of 26 can buy passes to travel on the whole subway, bus and train network in 2nd class only. Those who intend to spend some time in Paris, for their studies, work or enjoyment, can apply for the Émeraude or Améthyste cards if they are either above the age of 60 or 65, handicapped, war veterans or widowed by the war. The cards can be obtained from the local City Hall and entitle the holder to travel free or at half price, depending on the circumstances.