It's impolite to say "You're cute." to people much older than you. But it's fine between close friends. ^ ^

When you say it to your friends, you can say "귀엽다" or "귀여워" You should say "귀여우세요" "귀여워요"to elder person. However, if you say it to elders, you may be spoken ill of by he or she ;)