I myself learned 2 languages as a child, and taught my children the same way. As a result I am completely bilingual. I would probably teach 2 languages to a child up to the age of about 5. At the age of 12 however I think taking on more than one language would be incredibly difficult, especially as you will have a lot of other schoolwork to complete. I would pick one language and learn to the level you want to be and then move on to the next.