We are leaving for Paris in 3 1/2 weeks. In my research on museums and the metro I often see a mention of a discount for seniors. Are such discounts only for residents of France or member of the EU or are they also available for all visitors.

What constitutes a Senior and what ID's are needed?

As a certified senior it is my experience that such discounts apply to EU residents only. My attempts to obtain such have not been a success. But I don't stop asking.
I've seen than on the SNCF site but as I recall they're hard to get or cost money.

That's what I thought, but I know people here are a lot more experienced than I am, so I knew if anyone would had an answer it would be her.

Thank you for your help.

I had booked a hotel directly through the hotel's own website and then well before the date, I e-mailed and inquired if there were any discounts, such as for seniors, that were available, and got a response offering 10% off.
Don't know if it was specifically for seniors, but was nice of them. So do ask at your hotel, at least.

Thanks, we are renting an apartment so I was thinking of it for the Museumpass and things like that. It never hurts to ask just in case we can qualify.