May be anecdotal but my family is friends with two parents, the father French and the mother American. They and their children lived a few years in France and Montréal, and they spoke the two languages at home. I remember two of their children (around 8-9 years old at the time), not being really fluent in any language, but actually quite mediocre at both. They would start a sentence in french, struggle on grammar or vocabulary, and end up giving up and finishing it in English. I can't imagine what it would be like with two more languages.

Not really. I learnt my four fluent languages before 12. One was the mother tongue at home, the second was English, the third was the language of the region I was living in, the fourth was the majority language of my home country. I picked up French at 12 though it's not as fluent as the others (i.e. I don't dream in it yet).

Helped that three of the languages (and later French) were taught in school, I guess.