They don't actually say when Cheshire joined that I could ever find, but it would have had to do between when Artemis was nine to thirteen years old, so I'd say MAYBE when she was ten?  Possibly twelve at most, but somewhere those two times at least.  Can't remember what the year was for either, but Jade came back into Artemis's life when she was thirteen, and as her League of Shadows alter ego about the same time Artemis was running around as her own hero alter ego.  Hope that helps.