Is weight lifting safe for 15 year old?

yes it is, why are you weight lifting? for muscle toning or muscle building? 17 and 12 kilos wont build much muscle. Muscle mass is obtained by low reps and heavy weight. for muscle mass, try the "Lift to Fail" concept. Lift heavier weights till you just can't life anymore. Every day or every week keep adding more weight. For muscle toning, keep doing what you are doing.