너 생일 언제야? or 너의 생일은 언제야?

생일은 언제이에요? I usually use it when i asked to my korean friend and they understand

If you say to someone who is younger than you or friendly with you, say "네 생일은 언제야?" If you say to someone who is older than you or you must do politeness to, say "당신의 생일은 언제인가요?"

네 생일이 언제니? Keyword is '너','생일','언제'.