[accidentallfact]

People always overestimate how 'phonetic' their language is, because nobody actually uses phonemes in regular speech. In Korean in particular, there doesn't even seem to be any obvious correspondence between what is written, and what is actually said.

Foreign accents don't come from any inherent inability to learn language after X years of age. They come from people pronouncing languages as they are written, and virtually no language is like that in reality.

[DiogenesKynikos]

Foreign accents come from both.

It's true that when studying a foreign language, learning to read too early can harm your pronunciation. However, it is very difficult to learn new sounds that have no equivalent in your native language, and some languages have very restrictive phonology (like Italian and Japanese requiring a vowel at the end of every word) that their native speakers struggle to break out of.