If a 6 year old shot someone would the courts declare him a juvenile or would he be tried as an adult?
It is almost inconceivable that a 6 year-old would ever be tried as an adult, regardless of how heinous a crime they committed.
Juveniles can, of course, be tried as adults at times. However, most states do not waive a person's juvenile protections unless the person is 17. This age may be lowered to as low as 13 or 14 if the crime is particularly bad, but it is almost unheard of to go lower than that. (The case of Lionel Tate is one case where this has happened as he was tried as an adult for an act he committed at age 12.)
For most states, 6 is about the lowest age that a person can even be charged with a crime as a juvenile. Most states' laws hold that a person younger than that cannot even tell the difference between right and wrong well enough to be responsible for what they do in any way.
So, since 6 years of age is the lowest age that a person can typically be charged even as a juvenile, there is no way a child of 6 could be tried as an adult.