I think, if you have the ability to speak well enough in another language to teach a child, you should begin as soon as possible. If you start as early as possible the child should pick it up quicker as the child hasn't not known the language if the child is brought up with it. I think the older you get, the harder it is to learn another language as we start to reason with why things are a certain way and doubt ourselves. I work with children and there are a few children that are not originally English but as they were brought to live here young they are fluent in both languages and they get used to talking to their family in their language and everyone else in English. I am always trying to get the French child to speak french but she usually speaks too quick for me and I can't keep up enough to understand it all.