My first visit to GB was when I was 15 years old. I stayed there for two weeks with a friend and her family in a little village in South Wales and I didn't have any difficulties in understanding and speaking english. After having stayed there for one week I even thought and dreamed in english.
But this is very long ago. I still don't have difficulties in reading english texts, especially scientific texts, but I have only rare opportunities in talking to native english speakers.