I know a little Mandarin but not well enough to provide a true bilingual environment for my son. So I didn't actuallyy do anything about it until he turned 18-months. It was a hard slog because by then he was resistant to me speaking anthing exceppt English, so he's just keep saying "English" whenever I tried to say anythin to him in Mandarin. I've done a lot of reading on secnd lanngauge acquisition and it seems the earlier the better is the rule. Fortunately I found a good teacher for him and now he loves the weekly mandairn lessons with two of his other friends (who also come form English-speaking only homes). And, mood permitting, he'd even let me read him mandarin rhymes or sing songs in Mandarin. And on very good days, he would even concede to tell me names of things in Chinese!! So if you're serious, find a good teacher, preferably one with expeirnece with expat kids. It makes a whole world of difference!