Should a 16 year old be charged as an adult if they weren't attacked first?

This is a general response based on the limited facts presented in your question.

If the 16 year old "fought back" and used more force than was reasonably necessary to defend against a "shove", then yes, they could be charged with assault.

If the 16 year old was actively assaulting the 21 year old while another person is also assaulting him with a rifle, breaking his ribs, then yes, he could be charged as an accomplice to the rib breaker. The state would have to be able to show that the 16 year old knew and was there to assist the rib breaker in assaulting the 21 year old.

If the state decided to charge the 16 year old with Assault 1, then he is automatically charged in adult court based on his age and the charge. He can't be charged with Assault 1 in juvenile court.

If he has no prior felonies, his standard sentence range in adult court would be 93-123 months in the department of corrections. That's 7 years and 9 months to 10 years and 3 months.

Whether a child is charged as a juvenile or adult depends on the nature of the charge, not the defense. So while the 16 year old may have a valid defense, that will not affect where the case is brought. In Washington, Assault 1 is a "mandatory decline" case when a juvenile is 16 or 17. This means juvenile court has to decline jurisdiction and it is filed in superior (adult) court. If the 16 year old does not have a lawyer, he should get one quickly.