We did leave it till yr 11 before we realised what a problem it was becoming. I guess we had a little bit of a head in the sands approach and we are generally relaxed in our parenting approach.
General pattern was come home from school straight on to PC (online strategy military game) would surface to eat and play till we said bedtime. By year 11 this was impacting on school (attitude and grades), attitude towards us. Over the summer holidays between yr 10 and 11 ds son had started playing through the night. At the beginning of yr 11 this continued without us being aware. We only discovered this accidentally one night when I had got up with a sick dog.
It was horrible and unpleasant putting change into action but I could just see him failing his exams, ds could not see this at all, only now accepts it was becoming a big problem.
At first we were subjected to really horrendous tantrum like behaviour which stepped up yo being very aggressive verbally and lot of door slamming, stomping arround and stonewalling us. This is when it really struck home how addicted ds had become and this was a shock.
Ds still games a lot, it aside from one evening club he attends his only hobbies. generally has never been very sociable in terms of going out, is not sporty in anyway. We live in a tiny community (very socially isolated, ds largest class in school has 7 other students.).
I think now we have broken the cycle is doesn't cause him anguish to not play nonstop. We can now say lets go out for the day and ds joins us, he will also come to gym, shops etc.
I think as others have said it is important to find what motivates your ds addiction. For our ds it was a combination of boredom and a need for distraction, stress reliever. Ds recently watch all episodes of House, this may sound a little odd but we were ecstatic, this really was a sign of a complete sea change.
Ds is also now doing well in a levels - doing what he enjoys rather than subjects he felt he had no choice about.
Sorry for such a long post.