There's also a lot of much more recent, in-depth, and relevant study too, which directly addresses the 'critical period' hypothesis.
It's nothing to do with the ego. It depends on a child's neurological development, the most significant stage of which happens during adolescence when the brain is undergoing significant neurological pruning. An obvious example of this relates to specific sounds (phonemes). Take, say, the approximant /r/ phoneme in English: for learners whose L1 doesn't include such a sound, there will be a point in development after which the learner's ear loses the ability to distinguish it from other similar sounds (think: the typical difficulty Japanese speakers have with /r/ and /l/). And if the brain can't perceive the quality of the phoneme, it will never be able to produce it.