Make Your Way to the Cathedral Ward and Defeat the Boss
In order to access The Old Hunters you’ll need to have made it to the Cathedral Ward and defeated Vicar Amelia (that huge, white, fluffy, healing, puppy-like boss that is deadly as all hell). If you’re just starting the game for the first time, this may take you a while. This boss is not to be trifled with, especially since she has the ability to restore her own health. You’ll need to get past the first few bosses before reaching her, as well, so good luck.
If you’re on NG+ just make sure to make your way over there before the DLC goes live. You’ll need access to the relic in the back of the room if you want to try your hand in Bloodborne’s newest area. If you’re still on your first run through the game and have already passed this then you’re fine. Just make your way over when The Old Hunters goes live and you’ll do just fine. Be warned, though, if you’ve done the steps for Eileen the Crow’s questline you may have to deal with a powerful hunter in the Cathedral, meaning you won’t be able to fast travel there. So go check that out first.