At the age of ten i had the first feeling that my knees were hurting. If I had to run it went okay but after I did a run a pain came and it took a few days before the pain was gone. I went to the doctor and he told me that it had something to do with growing. His advise was to take some rest for a few months and than I could try to sport again. And i did and he was right because also when I did some sports my hurting knees did not came back. It is a little bit strange but now I am much older I feel that pain again and now it can not be because of growing. This can be arthritis but the doctor will tell me.