I only started to speak English when I went to school! My parents did the same and only spoke Portuguese at home. I picked up quickly and now I'm very thankful that I'm fluent in both! I only wish I was able to do the same for my DD....it's harder now that we only speak english at home, but she understands everything my parents or inlaws say to her (All of them except my mother speak VERY little english). We hope to put her in a 'after school' program to learn the language better.
So its doable at that age and kids are smarter than we think!