Poor Cullen, even if you didn't romance him he could be a good friend and Commander to see him fall so far is so sad!
*LOL*....I see Bioware is trying to pull on the heartstrings...
Cullen shouldn't be suffering from lyrium addiction/rejection at this stage since he's way too young. We've run across multiple older templars like Gregor and the blind Templar from origins. The one Templar we saw in origins that actually was suffering from lyrium overexposure was depicted as much older in front of the chantry. (The Templar who was the borther of the Bann of the waking sea in Howe's dungeon was suffering from lyrium withdrawl...slightly different affliction).
Cullen has probably been on the juice for a little bit long than Alistair has been a grey warden and in origins, I was given the distinct impression that the after effects of lyrium dementia was about the same time frame as the warden's calling - about 25 years.
Furthermore, this reflects VERY badly on any of the Divines since it means that unless Cullen gets off the juice, there's NO sanatorium re-established for older Templars? Even Leliana just ignores the issue - same leliana that's all about providing a shining example of how the chantry should act - yeah, kicking to the curb your most faithful servants who sacrificed their minds for you kind of contradicts this...
Uh, addiction can happen at any stage of someones life. The older templars suffered from a life long use of lyrium. Cullen even states in the game that you can loose your memories over time(that its subtle), it happens very slowly and some don't even notice and in the end you end up like the old templars we see in Origins. So the moment a Templar takes that lyrium they are addicts, its an extremely addictive substance thats how the chantry maintained their control over the Templars because they were one of the few reliable sources of lyrium.
Somehow I don't think the folk of Thedas knew just how badly lyrium could affect the minds of Templars, or that the faithful just went along with the idea's of Templars because its the Chantry that taught them that mages were evil and needed to be locked away. The Chantry perfectly constructed this series of events that allows them to maintain absolute control, and where would the templars go? They afterall were the only source of reliable (Free) lyrium, and most Templars are dedicated to the chantry, believe in the chantry and as Cassandra states "chain themselves" to the chantry.
Cassandra rebuilding the seekers and Cullen breaking free of Lyrium is the best option for Templars, the templars can become Seekers if they wish and if not Cullen takes care of Templars who wish to break their addiction or have fallen to their addiction. As for Leliana yeah she's contradictory!