If possible, learn it young. I learnt English at the age of 10 (which was best not to have any negative influence on the primary language as well), when I stay

If possible, learn it young. I learnt English at the age of 10 (which was best not to have any negative influence on the primary language as well), when I stayed in Australia for 3 years only but it stuck with me even after 20 years when my parents got much less confident in the language today. (Partially thanks to Internet where I could expose myself to English daily since then.) As for pronunciation, I can still make converstation with natives as I kept reading English vocally when I read online materials to this day. I think it's very important to keep your tongue and mouth remember the flow by actively speaking if you are no longer in a position to talk with natives. That plus the fact knowing the culture and the people in foreign country really makes a difference as an adult because you're no longer "afraid" of them (I especially feel this attitude in my country as a Japanese), those experiences have been such huge additions to my life, I'll be taking my kid abroad at certain age when I get one.