I thought my hairline was maturing so left it for 2 years until I noticed recently that I now have thinning that is worse than it was back then so I recently got on finasteride. I had the same hairline as you at 19 though so mine would be more aggressive hair loss. If you don't want to get on finasteride or anything yet I can understand, if I was you I'd take pictures of it every month and see if its getting any worse, if it is get on finasteride and you should maintain your hairline for many years. imo the mature hairline thing is mostly people in denial because when you first notice hair loss you can't believe its happening to you.