John Constantine debuted in the pages of Swamp Thing, which was then a part of the main DCU. However when he got his own spin off title(Hellblazer) DC had being making moves to create a more adult imprint for some of its more edgy or occult themed characters. So Constantine, along with Swamp Thing, Sandman and others reformed under the Vertigo imprint.
Now Bleeding Cool report that Constantine might be rejoining the super heroes..
..But now I understand that Constantine will soon be appearing in DC Universe titles. There’s a possible appearance in Flashpoint:Lois Lane And The Resistance looking at the cover, but what I’m being told is a longlasting return to DC Universe continuity.
As it stands, JC is currently in his 50s since
Hellblazer is one of the few comic titles that actually takes place in real time. It's unknown if this change will retcon that or keep the sarky old git as he is. More info as we have it.