> Most people will only ever be able to speak a language as fluently as a native speaker if they learn it before they're about 13-14 years old.

Very few people both have a ton of exposure to a language and actually study the grammar and stuff as adults. If you don't learn the grammar you will still speak like a dog after living in a country for 20 years. A lot of people in an average company don't write hard things at their job, didn't read any textbooks etc. and spend loads of time in meetings etc.