Anytime, really. There is no real bad time. The longer you wait, the more events you miss and the longer it takes to level your characters to a good point where you can do events easier. And they have at times given bonuses to people who "has owned an account before X date" like anniversaries and other things. So really, the sooner the better, just be aware that you should focus on getting your team up mainly and not as much on events.