To learn a new language relatively easily, you have to have the opportunity to use it. By using it more and more, you become familiar with it, and thereby become more comfortable with it, picking its nuances. But middle age can be a substantial barrier. My elders were comfortable with English, having learnt it in colonial schools; my father was even able to toss out an apt quotation in English at appropriate times. Yet, under the Japanese Military Occupation of Malaya, he had experienced great difficulty with the Japanese language.
In contrast, those post-war European immigrants who had arrived in Australia without any knowledge of English, were able to learn enough of the language, through continued usage, to be reasonably facile, if not competent, in a reasonable time. Those of us from Europe and (later) from Asia, who had learnt English before arrival, had only to moderate our accents – apart from coping with the unfamiliar Australian accents, and some quaint (and often colourful) colloquialisms. Yet, no matter how hard we tried, there were those Aussies who just had to comment upon, even denigrate, our accents, and our use of the language.
While I had learnt English in normal circumstances, coping with the idiosyncrasies of the language without much thought, the Indian shopkeepers in Singapore attending an English language evening class at which I taught a few years later, could not understand why the English language is not phonetic, like their languages; or why certain words which had the same spelling had different meanings (eg. bow, row); or why certain words with similar spelling were pronounced differently (eg. rough, bough, cough; or teak, steak). They kept asking why, as they struggled to remember.
Fortunately, they would be only speaking the language, not reading it. I refrained from suggesting that the English might have the same problem with their own language, especially with their regional accents.