The real answer is that is different for different people (and races) **cough cough** but the generally accepted idea is early as 4 months or older. Archive: https://archive.today/opok0 From the post: >>A giggling toddler in a pink dress and matching headphones lies down on her back in front of a gigantic whirling machine. A pillowy headrest cushions her head. She seems unfazed as she’s slowly shuttled into the claustrophobic brain scanner. Once settled, a projection showing kaleidoscope-like animations holds her attention as the magnetic resonance imaging (MRI) machine scans her brain.