The problem with SC2 is that it is actually a sport: if you don't practice regularly, your performance drops a lot. You can have a great strategy, but if you get supply blocked in the first 5 minutes because of an execution flaw or you neglect your base when you're attacking the opponent mid-game, the strategy may not matter. I play the game from time to time, but in bursts of a few weeks of playing several times a week followed by months of not playing at all.
Another factor that makes the game less attractive to players over 30 is that it is quite intense: you need to keep focused throughout the entire match. It is probably not the most attractive choice of game after you come back home from a day at the office. It is also not something you can play just before going to bed.