Firstly, it's important to say that it's never too late to learn anything! As the saying goes, you can never learn too much, and we don't believe that there are age limits to starting on the path of knowledge.
However, if we're talking about an optimal age for learning a language, most linguists agree: the sooner the better.
So what is the best age to learn a new language? Many studies show that there is a critical period for language acquisition which lasts from birth up to sometime between age 5 and puberty.
While there is much debate about exactly when the critical period for language acquisition ends, recent research suggests that up to about 10 years old is the age window within which native-like fluency and pronunciation in a second language can develop. During early years, the brain is more plastic and a child is unconsciously able to pick up the nuances of pronunciation, marking the difference from speakers who learn later. It's also a period during which children can learn languages almost without effort, and, critically, are able to internalize grammar rules without consciously learning them.
Simultaneous and sequential learning:
It should be noted that there is a difference in language development between children who learn two languages at the same time (simultaneously), and children who learn a second language after their first (sequentially). When a child learns languages simultaneously (which usually happens between 0 - 3 years old), the learning process is similar to a monolingual child. However, when a child learns languages sequentially (usually happens after 3 years old), the learning process is different and more influenced by their environment and personality.
Although older children may seem to progress faster than younger children in the initial stages of language learning, they usually don't achieve native-like fluency or pronunciation. While vocabulary can be learnt at any age, grammar seems more difficult to master as students get older. While it's possible to learn grammatical rules, they have to be taught explicitly and are difficult to internalise. In addition, self-consciousness, embarrassment or fear of making mistakes often act as blocks for older students learning a second language.
Of course, as with any learning, the speed and proficiency with which a child picks up a second language will vary. Particularly for sequential learners, many factors such as personality, natural ability, the frequency and quality of exposure to a language, and the interest a child has in it will all influence the rate at which they learn and the eventual level of fluency.
This is why it's so important to create a positive association with a new language and make learning fun.