Usual cause of knock knee in growing children is vitamin d / calcium deficiency. Sometimes injury , infection and occasionally no cause. At 17 years , the bones are almost fully matured like an adult and correction requires ( mostly for cosmetic reasons) surgical correction by changing the angles of the thigh bone near the knees , usually done one knee at a time.

You need to get an xray done and vitamin d3 levels to look for the degree of knock knees after that we can think about correction