[duado]

Learning a language as an adult is completely different from learning as a kid. Your hypothetical “dumbest French person” would not have been able to learn French as an adult, the same way that the most physically fit 100 year old could not survive the falls down the stairs that 3 year olds do without even crying.

[dorfsmay]

Correct. A different part of the brain is used when learning a language past a certain age (I think 8 or 10 years old).