Yeah, I know. It's the "The sooner the better" mantra. Unfortunately, it only works in a total-immersion-type natural setting. From my experience, the best age for kids to start learning a foreign language in a formal or semi-formal setting is approx. 8-9 years old. They learn much faster and are way more motivated.
unfortunately there is no way to prove this. the school my kids go to is half polish half english- 75% of the students are Polish and only speak Polish at home but learn English and often French with no problems.
Children are able to assimilate up to 6 languages by the age of 6. This doesn't mean there will not be problems along the way- but ultimately who gives a rats' if a child is slower at spelling or reading when the ultimate outcome is speaking more than one language.
For example, my 7 year old has been having a hard time with her reading. We read each night for 20 minutes and her reading has improved. I don't care if her reading is poor at 7 when she speaks two languages and is learning how to read and write- ultimately by the time she reaches middle school is she is still having problems then I would be worried. If a child speaks more than one language there is bound to be problems compared to a child that is unilingual.