The formats used in the 2016 Season of the Pokémon TCG has been announced today on the official Pokémon website. As suspected, the 2016 Season's Standard Format will be "XY-On", meaning all Pokémon TCG sets starting from the Pokémon TCG: XY set are legal for Standard Format decks — older cards are not allowed in Standard Decks, but the Expanded Format has been confirmed to remain as "BW-On", so cards from the Pokémon TCG: Black & White sets onward are legal for that format.
Though the 2016 Season has officially begun so players can earn Play Points and Championship Points at premier tournaments now, the "rotation" of these sets will not take effect until September 1st, as per usual. This means that the Pokémon World Championships will still utilize the existing 2015 Season formats, along with all other tournaments until September 1st.
Most local tournaments go by the Standard Format, so several Pokémon TCG sets have been "rotated" out for this year's Standard Format to help keep the gameplay fresh and encourage people to come up with new strategies. Some tournaments will play in the Expanded format, and Regional and National-level tournaments play Standard the first day and then Expanded the second day (allowing players to change their decks between days). You'll want to check with your local Tournament Organizer to see which format their tournaments are using to keep your deck legal.
You can find out more information about this change, including all of the Pokémon TCG sets that are legal in both formats, over at the update on the official Pokémon website.