I challenge you to go to China and ask people to make fun of you if you are unable to correctly pronounce half their words. Not because of stupidity but because of a mix of not hearing the subtle difference ("but that's exactly what you said!") and being unable to accurately reproduce a sound that you hear.

As kids, we have the ability to make lots of noises. Kids learning languages keep those skills alive. Over time, we lose that ability for sounds that we don't use regularly, and re-acquiring that capability is really hard.