To answer the first question, we can't help when we start liking guys. It is all apart of growing up. Some girls may start liking guys earlier than others. I don't think there really is a specific age to say that it is okay for a girl to like a guy. If you remember when we were little and we played with boys, and it wasn't a big deal. Then as we all started growing up, it became weird to spend so much time with boys. Its just a part of growing up when you start liking a guy, it isn't wrong to like a guy when you are 10 or however old anyone is.
Now for the second question, I want to start out and say we can't really help if we have feelings or like someone. We are humans and will have feelings for certain people. I don't think its wrong to like boys or have feelings for them. I don't think its necessarily healthy if you are obsessed about guys though. As girls we can get caught up with which guy is the best looking, who is the nicest, and who has the best sense of style. All of this is good, but we need to leave room in our life and hearts for Jesus. If we allow guys to become the most imporant part of our life, we won't leave room for Jesus. We need to look to God for our happiness. We shouldn't look to guys to make us happy. When we can be happy with ourselves when we are single, will be the time that God can do the most work in us. We have to be able to put our future relationships in God's hands. He will let the right man come into your life when the time is right.
I hope this helped any of you guys that had questions about this. If you have any more questions please feel free to ask. And if you would like any advice on a specific topic, please feel free to go to my Advice Column Page. (you will always remain anonymous.)