Skill Challenges account-wide?
No, it means that if you unlock them on a low level character, all your characters can see them from level 2.
Right now, you need to be level 13 to do your first skill challenge. It only affects people making alts now. And after this patch it only affects your first alt.
No, what it means is that once you have one character get high enough level to unlock the ability to do Skill Challenges, all other characters on your account can do Skill Challenges even if their level isn’t high enough for them to be unlocked if it was your first character.