f(x) member Krystal revealed a young picture of her online.

Today, Krystal posted on her twitter, "Thank you everyone for your birthday wishes. Everyone's saying that now I am finally 20, but I was just 17 a day before. I want to stay this age," along with a picture.

In the picture, Krystal's baby picture was revealed, showing off her doll-like beauty.

Internet users commented, "Happy birthday" and "She's just as pretty now as she was back then."