I have read a tiny bit of the literature on language acquisition, and what I have read says that mere exposure to the phonemes of a language (even absent any attempts to actually teach that language) during early years (definitely before puberty but with greater effect between the ages of 1 and 6) makes it easier to form those phonemes later in life.
That said, I think every second-language learner (even those who will never learn a third language) could benefit from being taught the entire international phonetic alphabet, to aid them in mentally placing (inside the mouth) the sounds they're supposed to be making.
Knowing the difference between 'a' (unrounded open front) and 'ɐ' (unrounded near-open central) vowels helped me when learning the Cantonese phonetic system. Just knowing all the sounds that are "available" and knowing which of those sounds the target language uses is very useful.
That said, I think every second-language learner (even those who will never learn a third language) could benefit from being taught the entire international phonetic alphabet, to aid them in mentally placing (inside the mouth) the sounds they're supposed to be making.
Knowing the difference between 'a' (unrounded open front) and 'ɐ' (unrounded near-open central) vowels helped me when learning the Cantonese phonetic system. Just knowing all the sounds that are "available" and knowing which of those sounds the target language uses is very useful.