In casual language, - 너 영어 잘해, 걱정하지마! In honorific language, - OOO씨 영어 잘해요, 걱정하지 말아요! (OOO=His or her name) Even the person you talk to is the same age as you, if you two are not so close enough to use casual language, honorific language is still able to be used in many cases. So, I answered for both two versions. Hope this help you!

The way you said it is fine! You could also say 'Your English is fine, so don't worry', but your first answer is correct too.