You just heard wrong on the Navigo. The products available to tourists do not provide discounts, either for seniors or for teens who are not residents. Locals have Navigo cards and. there are senior cards and student cards for kids. But. for tourists you have the Navigo Easy for individual tickets where children 4-9 get half price tickets but 10 and up are full fare and day passes . And the Navigo Decouverte is available for daily, weekly or monthly passes. There is no child or senior version of these. There is no reason local tax payers should provide discounts to people wealthy enough to travel internationally -- they do provide discounts for their own youth and seniors.
The place we did find lots of senior discounts for tourist visitors was Spain and there are a few places in town where seniors get a one or two Euro discount on admission tickets. The Jeu de Paume is one and I have had those discounts at other photo exhibits.