I am 18 , having slight bow legs , i don't know what is the reason of it, never had it when i was young , began to notice it last year, any treatment ?

Bow legs: The alignment of your knees is how your skelton formed as you developed. In order to change the alignment of the bones you would need an osteotomy where the bone is cut and a plate & screws or other device is used to hold the new alignment until it heals in about 8 wks. Unless you have knee pain from your alignment it may not be for you.

None necessary: There's no practical treatment available. Your legs will most likely normalize naturally.

Hmmm?: Hard to say, but if your alignment is changing and you have stopped growing you should really get x-rays to check things out.