You are being charged for breaking somebodys jaw what consequences can you face if both parties are 17 18?

Jail, prison and/or probation, the length would depend upon lots of factors, including the state in which the crime took place. Also if they charge the offender as an adult rather than a juvenile (at 17/18 it's a pretty good bet it will be as an adult). Civil liability would be medical costs and compensation for pain and suffering and humiliation.