The critical period idea says native-like fluency needs learning:

1. The hypothesis claims that a person can achieve native-like fluency only before a certain age. 2. It links language ability to the developmental stages of the brain. 3. So the answer is before a certain age. **Why not the others:** Learning after school years is the opposite claim, and lessons or drills are teaching methods, not ages. _Source: Wikipedia, "Critical period hypothesis", section "introduction" (revision 1375020650)_