Couldn't find any info on google to confirm from blizzard about the rifts but I did read:
Quote
Level your seasonal with Adventure Mode: You might think that the “Like a Boss” objective can’t possibly be real, because you’ll kill the Butcher in Campaign Mode before you kill any of the others. However, Adventure Mode was open to my seasonal PTR toon before she even entered her first game. The other objectives point to being able to run bounties and rifts for leveling instead of being confined to Campaign Mode first. After questing a little in Campaign Mode I set off to confirm that new Seasonals will be able to run bounties and rifts at lower levels.
Although I grabbed the town portal through Campaign Mode first, I didn’t yet get to the templar, the first follower you normally get. When I arrived in Adventure Mode, all three followers were there for my level 11 seasonal witch doctor. They had their basic gear equipped, which means that the Enchantress is likely more powerful to start, as she has “later” gear than the other two.
Nephalem Rifts are available but Greater Rifts are still locked to level 70. I started a Nephalem Rift before I even had my whole action bar unlocked and I finished it three levels later. Even though I had only collected 14 Blood Shards in the rift, Kadala was standing there ready to offer her mysterious goods.
So that's nice I guess.
This post was edited by Legit-Trades on Aug 13 2015 03:03pm