Well actually this is a very good question.
It makes a tremendous amount of difference what your mother tongue is
when it comes to your ability to pick up English, especially
conversational English. For natives of the Far East region
(Korea, Japan and probably China too), their language structures are so
vastly different than English, it is a tremendous struggle to learn
English as a second language. Whereas, some of the European
countries speak languages with similar roots and grammar structure
which make it much easier to learn English.
My parents immigrated to the U.S. when I was just 13 years of age from
Korea so I had the advantage of being immersed in English language at a
relatively early age -- but it was, and still is, a big strugle.
Then when I was in high school (long long time ago mind you) I took two
years of Spanish. And it was a cinch!!! Not that I aced it
without studying...don't think I aced it at all...but my point is since
I already was pretty fluent in English, learning Spanish was so much
easier! I remember thinking "wow if I had to learn this language
from Korean, what a mighty struggle it would be!!".
I am told the easiest language for a Korean to learn is Japanese and
probably vice versa. And I know Spanish and Portugese is very
similar.
Now as far as being grammatically correct....well that's another
story. I have seen many a high school English teachers from Korea
and China come to the U.S. and not be able to hold a casual
conversation in a restaurant. Being able to read and write and
score high in TOEFL is different than being able to converse.
Finally, I agree with previous posters....the key is to start
early!!!! Kids have amazing ability to learn. Not only
langauages, but sports, musical instruments, arts, etc. Well I
tried my best...but now my kids are teenagers and...need I say more?!