I think it would be incredibly difficult to reach agreement on any "absolute limit" defining an age at which one ceases to be classified as a juvenile and becomes an adult. Brain research is showing us more all the time about how the areas of the brain develop and what the timeline is for that development. However, the "timeline" is not the same for all individuals, which is why that non-negotiable age is so difficult to establish.
The unfortunate fact is that young adults don't have the experience, the intellectual or logical reasoning capacity, or the foresight to consider consequences before they act. As a result, it's easy for teens to consider themselves as invincible and to act accordingly, at times with tragic results. When this happens, they learn about risk and consequences the hard way.
A very definite age requirement is needed in order to administer the justice system fairly. While the above posters make valid points about when, exactly, a juvenile might be considered an adult, get ten judges to rule on the case and you may well be split 5 - 5. If the age limit is set at 18, as an absolute limit for being tried as an adult, then the 14th Amendment right to "equal protection under the law" is protected.
The issue goes far beyond that expressed in the above post. The age of the accused is but one factor to be considered in whether or not he/she should be tried as an adult. The seriousness of the offense is a factor, as is the ability of the accused to understand the nature of the offense and the wrongness of it. His ability to be rehabilitated is also a factor. In most states, if one is tried as a juvenile, he cannot be incarcerated past his 21st birthday, regardless of the nature of his crime. At times this is simply insufficient punishment, and it is necessary that the juvenile be tried as an adult and thereby subjected to more appropriate punishment. If tried as an adult, he has the same protections and presumption of innocence as would any other accused. He simply doesn't get an "easy out.""Life Decision" normally are not a factor in culpability for a crime.
Many states allow juveniles to be adjudicated as adults if they are over the age of 14 and have committed violent crimes. At 14 most juveniles understand the difference between right and wrong, however the center of the brain which controls impulsivity has not fully developed. It is possible the violence and impulsive nature of a juvenile offender would not necessarily be present in that same youth in years later. Unless we can present a new system in which youthful offenders may be incarcerated past their 21st birthday and yet not be treated as adult offenders we will continue to adjudicate juveniles as adults for those violent offenses. We cannot allow certain offenses to go unpunished in our society regardless of a persons age.