My family didn't emigrate out of Taiwan to New Zealand until I finished 4th grade so I can speak and read mandarin just fine. However I have forgotten how to write it and the only words I can write are my name and the numbers. Ironically my mother was an elementary school teacher in mandarin and held tutoring classes in Auckland to this day. She just didn't teach me or my brother.
Can still type it though.
Also took Japanese in high school only because my mother wanted me to. I wasn't motivated so never got past the basics despite 3 years+tutoring. Ironically these past few years I have been picking up more Japanese than I ever did in school because I'm into anime and I've been steadily learning just by listening the language
. On top of that chinese and taiwanese gets a natural advantage by being able to read kanji.
Interesting thing about accents, my brother who's only a year older has a noticeable Taiwanese accent yet mine is pretty much kiwi. So it appears that the cut off age for accent appears to be 10.
Mom also has pretty atrocious english, and she claims that she was top of her english class during uni (nobody believes her).
With the internet maintaining a language is pretty easy, just browse forums in that language from time to time.