I am a senior female solo traveller, I'm going to Paris for the first time in September for 4 days. Is it best to get the IDF tickets or the Paris pass?
How would I validate the ticket? Do I need photo for the IDF ticket?
“Paris Pass” appears to be a third-party product that bundles the official Paris Visite pass with a few extras. Given that Paris Visite is rarely worth it, because it's a lot more expensive than passes for just transportation, it's probably not worth it either, but it depends on how much you'll travel. When passes advertise huge savings, keep in mind that this assumes that you'll make use of every single discount that it offers, even though you wouldn't have either the time or the inclination to visit more than a few.
As far as I know, senior discounts for Paris transportation are only available as yearly passes, not for tourists. So you'll have to pay the normal fare.
The daily pass is called Mobilis. Its cost depends on the zones. The Paris municipality itself is zone 1 and contains most attractions. Versailles and Orly airport are in zone 4, and Roissy CDG airport is in zone 5. You don't need a photo, you just write your name on the pass and be prepared to show a passport or other photo ID.
A weekly pass is considerably cheaper than 4 daily passes (€22.80 for all zones), but is valid from Monday to Sunday only. It can only be loaded onto an electronic Navigo card which you need to get from a ticket window for €5 and a photo. The total is still barely more than one day of Paris Visite that includes Versailles and the airports. Although this is considered a commuter pass and most literature about it is in French only, anyone can buy it (the only advantage locals get is that we don't pay the initial 5€ for the card itself).
If you don't travel a lot, point-to-point tickets for the airports and wherever you go outside Paris (for most people, that's only Versailles) plus métro tickets in Paris (bought 10 at a time: a “carnet”) may be cheaper, especially if your stay would overlap two calendar weeks. A pass does give better peace of mind.