I am 9 years old now, and a girl told me that I love you. Is it true?/You don't say.

First of all, she may be a lively and lovely little girl. Openness is her characteristic. It is normal to say "I love you to death" easily, because it may be her mantra. Second, if the average child really wants to develop a "long-term" relationship with you, it may be because he relies on you rather than likes you. On the whole, you'd better continue to be friends!