As long as the ticket says “ticket t+”, it's valid. (Official source: RATP FAQ — I can't find an official English translation, but that's what the French says.) It doesn't matter how much you paid for it, only that it's a ticket for the right kind of transportation. Buying a carnet of 10 tickets is cheaper than buying 10 individual tickets separately, but they're the same tickets, valid for exactly the same things.
Older metro tickets (sold before 2003) are officially no longer valid, although for all I know their magnetic stripe still works in the gates (but if you get inspected, you can be fined).
There is no expiration date for Paris suburban train tickets either. As far as I know, for a suburban train ticket, as long as it's the right physical format with a magnetic stripe, it's still valid. Most other train tickets in France do expire.
Regarding zones, they're irrelevant for travel in the metro. With a ticket t+, you can make any metro trip, the ticket remains valid as long as you don't exit the metro. There's a 2-hour limit, which is sufficient to reach any station from any other station (if you want to joyride, get a day pass).
Zones are only relevant for suburban trains, including the RER. A ticket t+ is valid on the RER as well (you need to put your tickets through the gate when changing between metro and RER), but only inside zone 1, which is the Paris municipality. This means that for example you can go from Paris to La Défense with a ticket t+ using metro line 1, but if you want to take RER A then you need a point-to-point ticket because La Défense is outside the Paris city limits.
Incidentally, a ticket t+ is also valid for a bus trip, but you can't combine rail and road in the same trip. Tramways count as road: you can do a bus+tram trip with a ticket t+. Ticket t+ is valid on most buses in the Paris region, but not on a few long-distance buses, notably the airport buses.
Via Navigo is an official site of the authority that manages transportation passes in the Paris region. It's giving you the price for a single ticket. If you dig through and you speak French you can find the price of a carnet of 10 tickets there as well.