It is safe for a teenager of 13 years old to take marine collagen, but due to fish being prominent, a parent should ensure the teenager doesn’t have any allergies. As teenagers are still growing, they are producing collagen naturally, so they wouldn’t see the obvious benefits as much as someone in their adult years. It would not do them any harm and could help keep their bones and joints strong, especially if they are physically active by playing sports, dancing or horse riding etc.