A fresh DK is lvl23 (normal races lvl55 atm) or lvl24 (allied races lvl58 atm) post-squish
Anyone on a connected realm. Each individual server counts as its own realm.
No matter if you want to play them or not, leveling a Death Knight allied race is the quickest way to get their heritage skin.
Don't people who still burn their time in wow have way too many max level characters already?
Am I the only one who ever wanted to make a class 1 DH? Think how much FUN they would be to level!