This is, of course, a matter of opinion. There is absolutely no way to answer this objectively.
The most common answer to this is that some minors should be tried as adults. There are generally two criteria used for when minors should be tried as adults. These criteria are the age of the minor and the gravity of the crime.
Many people feel that minors who commit really terrible crimes should not be able to get off any more lightly than adults. Very few people want minors charged as adults for burglary. Many people want them tried as adults for murder. In addition, many people feel that older minors should be more likely to be tried as adults than younger minors. A person who commits a murder within a month of their 18th birthday seems much more culpable than one who does so at 12.
So, most people would argue that minors should be tried as adults when they commit really serious crimes in their late teens. In those cases, the feeling is that they had the capacity to truly understand what they were doing and should be punished accordingly.