Being bilingual brings with it many benefits and the benefits vary depending on the languages you speak but also on the country you are in. There is for example not much benefit in being bilingual in Spanish and Malay when you live in Switzerland. Unless you can of course find a job with Spanish or Malay tourists, the drawback is that you need to be able to speak Swiss. Switzerland is probably not the best example because most Swiss people are bilingual already with all those borders around. Many of them speak Swiss-German, Swiss-French or Swiss-Italian. However I am sure you get where I am coming from.
The best time to learn is when you are young at least that has been my experience. I have been living in a foreign country now for 8 years and I arrived speaking only English. While I can now speak some of the local language I am by no means bi-lingual and I think most of that relates to the fact that I am too lazy now at this stage in my life to learn properly. I havent taken any courses in the language either so perhaps that is part of where the problem stems from. However, I digress.
Learning when you are young has some great advantages in that you are already learning one language and when they get all mixed up children tend to take the information in easily. There are pitfalls in that sometimes the children have trouble deciphering which words come from which languages but usually in the long term this will rectify itself. It does cause some confusion for the child however when they are not sure what language they are speaking and when they are dealing with adults that may or may not speak the languages they know.
It seems as well, with the children I have encountered here that after they have mastered the art of two languages they seem to be much quicker at adapting to more languages. At least they seem too much more quickly than other children who have only grown up with one language. The children who grow up learning bilingually generally have far higher levels of reading and writing skills and better social interaction skills. This does of course depend a lot on the environments they are exposed to as well.
Some of the other pitfalls however is that there can often be a delay in when the children actually start speaking. This is not always the case however but there are many times when children of bilingual parents or who have parents who speak different languages do sometimes start their speaking at an older age. But as said above usually when the speaking starts the progress at mastering the languages goes very quickly. It seems that perhaps the children are trying to process all the information and some how their mind is telling them to wait to try it all out until they have a better grasp on how it all works.
The natural course is that the child will speak using both languages as at an early age they are not sure which words go with which person or language so they do have a tendency to mix up the languages together. The trick here seems to be that as much as possible the parent should speak in their mother language with the child and even when they understand the mixed sentences being offered by the child they should reply in the language of their mother tongue with the correct wording. Not of course in a way that makes the child feel they have done wrong but just as further education for the child when they hear the correct sentence in the correct language.
They will figure it out it just takes a lot of time. Some people when they are older and are learning another language find that they start to count in the new language, think in the new language and sometimes even dream in the new language. So if that happens to adults imagine how difficult it is for a child to process all of that as well as grow and learn all the things they need to in life.
Having an education that is bilingual can help your child immensely in years to come so if you are of a couple that has more than one language between you dont stick to one, mix it up and let your child figure it out. If you are considering your children and their future then getting them started in their learning of new languages at an early age is a great idea. http://www.abso-bloody-lutely.com/Education/Others/Bilingual_Education/