A recent study shows that adults can learn languages better than children. There are other studies that show children as better at learning languages, and point to a critical period after which it is more difficult to learn languages. We know empirically that children, especially children under 10 or 12, become fluent in a new language more quickly than adults, with better pronunciation although often with a more limited vocabulary.
In fact it is motivation and attitude, not age, that determines our ability to learn languages. Most, if not all, polyglots, learn most of their languages as adults. Adults are often more inhibited or self-conscious, and have less opportunity, or are less willing, to socialize with people of another language group, whereas children just blend in to their new environment.
The only thing that matters is that we can learn at any age. If we are 50 there is no point in wondering if we were able to learn better when we were 5. If you can motivate a child to learn a language when young, great. Otherwise it is never too late to start.