Learning a foreign language is more than just learning grammar and vocabulary. It is also learning to distinguish the different sounds of that language and the ways to make those sounds. I can't do the searches for it right now, but from what I recall, people lose the ability to distinguish sounds that they don't hear often by a very young age, since their brain figures it's not important to distinguish those, so that it becomes increasingly harder to pronounce and understand those languages which use different sounds. This is why L and R differentiation is very hard for Asian speakers, and tonal languages like Chinese and Vietnamese are very hard for English speakers.