The original sentence is: The brain grows the fastest from 2 to 11 years old.
To make it grammatically correct and clear, you could say:
- The brain grows fastest between ages 2 and 11.
- The brain grows the fastest from age 2 to 11.
- The brain grows most rapidly between 2 and 11 years old.
Each version is correct, but adding "age" or "between" helps clarify the sentence. For a 6-year-old, the first option is simple and easy to understand:
The brain grows fastest between ages 2 and 11.