Update: According to recent announcement, the Carte 12-25 is no more; now it is replaced by two distinct cards, called Jeune 12-17 and Jeune 18-27; they basically offer the same benefits, and an extra year of eligibility is added in comparison to the original 12-25: it is possible to get the card until 1 day before the 27th birthday and benefit from it until 2 days before the 28th birthday.
The Carte 12-25 and the Voyageur program are two distinct things.
This rail card stil exists, and is still popular among young French travellers. I actually own one; it returned on its investment a long time ago!
The Carte 12-25 is the rail card that gives proof to the fare inspector that you are eligible for the discount fare, and have paid the yearly fee.
The Voyageur card is the rewards program, just like airlines do. Anyone can get a Voyageur card and earn points. It is also used as an electronic ticket support : you can associate your tickets in the "e-billet" format (nominative as opposed to transferrable paper tickets) and the inspector on board will verify them by scanning the 2D barcode on the card.
I see where you could be confused:
Back in time, at the beginning of the year, the Voyageur program did not exist. Loyalty programs were restricted to discount card holders : anyone getting such a card (12-25, Escapades, Senior, Enfant+) automatically got the associated loyalty card in the mail.
The loyalty program, called "Gagnez à Voyager", had the cards branded with the same name as the associated rail cards. Mine was a "Gagnez à Voyager - Carte 12-25". The earned points, called "S'miles", were valid among an alliance of shops to which the SNCF belonged to. For example, you could earn points in travel and get discounts at Monoprix supermarkets in return.
Later, the SNCF decided to quit the alliance and run its own loyalty program. No more Gagenz à Voyager cards, no more S'miles. Points were converted into the new "currency". The notable changes introduced were:
- No more alliance: points only work at the SNCF
- Anyone can join, no more restriction to rail card holders
- All cards are electronic ticket supports using a 2D barcode on back
The rail card still exists and has not changed. Of course, you can order one online. Note that it is very easy to get one from a physical ticket booth or travel agent too: you only need valid proof of your age (the passport is OK) and your card is printed on the spot for you. It is issued on the same cardboard as a train ticket, folded in three and placed into a small protective cover. A photo is usually requested but not immediately; if you do not have one when buying the card, the agent will give you the special transparent sticker to add it later. On board, you can show an ID with your rail card to give photo identification in such cases.
It is a wise investment for anyone eligible. One good detail is that, in some situations, you can get same-price or cheaper first class seats as compared to second class on reserved trains. This is due to how yield management works; 1st class and 2nd class quotas are separate. Very nice to benefit of that in France, since some other rail networks simply do not offer youth fares in first class.