OP I'm involved and around a lot of people who speak multiple languages.
I know people who studied a language at uni that they hadn't studied at school, or knew in any way before, and became fluent.
I know adults who learned a language due to living in the country it's spoken as kids and can barely string a sentence together as adults because they haven't spoken it in years.
We have diplomats who are taught new languages to fluency..and they're definitely not under 11.
The idea you have to learn before 11 in order to become fluent is nonsense.
What might be true is that if you learn fluency before age 11 (so you have the vocabulary a native speaker of 10 has when you're 10) that you'll have a native accent. That's also likely because you're learning from native speaker(s). The accent is something that can be hard to get when you're older, however, Im sure you've heard an Italian or French person speaking great English, but with their accent? You could still u defray and them? They can even be native level fluent, but have the accent still.
I would not stress about learning a language for a young child. Strengthen their English. The wider their vocabulary is in English, the easier it will be to learn another language when the time comes. And be open, as you are to other cultures. And if at all possible, try to visit the country on holiday (can be camping in a farmer's field, doesn't need to be fancy).
Then make sure kids choose a MFL at high school.
Don't stress about this. The 11 age simply isn't true in reality!!