Actually at 13 this is a great idea don't listen to retards telling you no, they fear the true power of the hammer.
But you need to take the risk of disfigurement seriously. The hammer is powerful indeed but you need to know how to control it.
The best places to smash are the zygos, chin, brow ridge and orbitals. Smashing the jaw can work especially the ramus but you need a different approach. The goal is to cause small microfractures and let it heal so if you don't want to overdo it.
Thats why you want to keep the volume low and intensity verry high. Zygos, chin and brow ridge are verry resistant to blows so you are gonna have to take full swings. But don't worry if you keep the swings to a maximum of 6-8 per square cm with the flat backside of the hammer you should end up with a couple of bruises you can cover up after with concealer. Also remember to always brace the back of your head against the wall so the skull takes the full impact. For your orbitals you want to keep the volume a bit higher and lower the force since the bone there is a bit softer. U take 50-100 nice gentle taps with the tip of the hammer (not the flat bottom) per square cm. Then for the ramus and jaw you are gonna need to use a verry special technique. The volume and power both have to be high, the problem is of you do it with a hammer you can damage your nerves severly. This is why you have to hit with the knuckles. A good 20-45 swings per square cm should do the trick. Also because you are gonna be smashing yourself from either the left or right side you want to brace the other side of your head to the wall instead of bracing the backside to absorb the power better. After the session you want to rest untill the bruising dissapears, should take about 5-10 days. Then repeat. Always use a quality steel hammer and wash your hands and face before and after each session. If there is blood, always check if you still have sensation in the area and if you can move your face. If not you are gonna have wash your face immediately with cold water and spread some vaseline to reduce swelling. Again you are gonna want to rest untill you can feel your face again, no smashing untill the swelling is gone kid. Saftey first. I have been doing this for a ferry long time not and I m comopletly ifne. Im doring in right now astualy. Anyway rememver always be cuer careful and neever overdoit. Wonc e you ger good at it you dont even fefeliit. If you have and mroe questinos go ahead.