Too many players take him jungle and then say they can't gank until 6.
A) This is bull shit, get red, proc it with your Q or gank the lane when they're low on health so your passive kicks in.
B) I think every champion should be able to do what they're supposed to before they are 6, so this leads to my next question :
It's less that they can't gank before 6, it's that they know that unless your lane has heaping amounts of CC that you save for when he appears, Warwick's ganks have no real way to either a) Get Warwick in their face, since his Ultimate is his main gap closer or b) Lock someone down, because he has no forms of hard CC.
The big problem behind pre-6 Red Buff ganks is that Warwick can't jump on someone like Vi or Lee Sin can and immediately catch someone from where they are standing into the Red Buff's effects. This means that in order for Warwick to significantly get the jump on someone at this point of the game, they most likely would have to be far, faaaar past the midpoint of their respective lane, which isn't something you can always bank on and is harder to do in higher levels of play as people tend to ward more the further out they are. On top of this being unable to jump on people, if your lane has no forms of Hard CC, there is no possible way for Warwick to get his DPS based damage in. Remember, his damage is DPS based from his auto attacks, not front loaded in abilities that AD Casters like Wukong or Lee Sin can do.
Ultimately, if your enemy laners do happen to be sufficiently pushed up enough to get a good gank on them, and if your lane has the Hard CC to actually lock someone down for whatever amount of time, then Warwick can and should take opportunities to gank that lane if he can. However, you can't honestly ask him to come gank your perfectly down the middle lane and your champion has no forms of Hard CC that you can use in a gank without knowing well enough that you are probably wasting Warwick's time pre-6.
tl;dr: Yes, Warwick CAN gank pre-6, just don't expect the gank to be any good unless you can set it up for him.