We do know that children who learn languages early have a lot of advantages. For example, there’s an interdependent relationship with English literacy development for children who learn languages early. There is also a greater meta-linguistic awareness, which basically just means the engagement with a second language makes even very young children a lot more aware of language, of its different roles, the structure of sentences and so on.
We also find that there is an increased mental processing capacity through the engagement with a second language, which leads to a better memory and better control over information processing. For example, children become better at ignoring not so important facts and focusing on the important things.
In general, there are many advantages to being bilingual. One interesting benefit, based on many research studies, is that knowing a second language can slow down mental decline later in life. We can also see long term benefits in the way that it’s easier to learn more languages if you have learned a second one early in life. Knowing a second language also instils a general appreciation of other cultures and better intercultural understanding.
What we are starting to see now is that most industrialised nations are training their citizens well in English, so their citizens become fully bilingual. Monolingual English speakers might soon be the only monolingual speakers in the world and might be competing for jobs with people who can speak English as a second language well, but also have another language.