I'm 18 years old and I haven't grown my chin/jaw. Will my chin develop without the help of braces? (Photos)

Your age is 18 years, so now growth is not possible with the help of growth modification appliances. so the only option in your case is advancement by surgery.

The chances of your lower jaw growing forward to the point that you would be happy with the way it looks are very unlikely, either by itself or aided. 18 year old males are close to the end of their skeletal growth.  You might have another 2-3 years of skeletal growth remaining. The thing is, everything is growing! Maybe the lower jaw grows forward, but so does the upper jaw and the rest of your bones.Definitely, more information is needed, but it sure looks to me like jaw surgery of some sort will probably be needed to give you better facial esthetics.Good luck!

When the lower jaw is not "growing" it is typically because the upper jaw is too narrow and therefore does not allow proper growth and development of the lower jaw. Think of the upper jaw as the garage and the lower jaw as the car. If the upper jaw (the garage) is too small then the lower jaw (the car) won't fit inside. Your lower jaw wants to grow further forward but due to the narrowness of your upper jaw it is unable to do so. More than likely (can't tell for sure without adequate pictures) your upper jaw requires development first. This is done with an arch development appliance. After the upper jaw is fully developed the lower jaw will often come forward on its own; if it doesn't, then a lower appliance that encourages forward growth of the mandible is used to bring your lower jaw forward. Good luck!