IANAL, however-
The age of reason- The age at which a child is considered capable of acting responsibly.
Under Common Law, seven was the age of reason. Children under the age of seven were conclusively presumed incapable of committing a crime because they did not possess the reasoning ability to understand that their conduct violated the standards of acceptable community behavior. Those between the ages of seven and fourteen were presumed incapable of committing a crime, but this presumption could be overcome by evidence, such as the child having possession of the gun immediately after the shooting. The rebuttable presumption for this age group was based on the assumption that, as the child grew older, he or she learned to differentiate between right and wrong. A child over the age of fourteen was considered to be fully responsible for his or her actions. Many states have modified the age of criminal responsibility by statute.
All states have enacted legislation creating juvenile courts to handle the adjudication of young persons, usually under eighteen, for criminal conduct rather than have them face criminal prosecution as an adult. However, a child of thirteen who commits a violent crime may be tried as an adult in many jurisdictions.
-- West's Encyclopedia of American Law, edition 2. Copyright 2008 The Gale Group, Inc. All rights reserved. (from http://legal-dictionary.thefreedictionary.com/Age+of+Reason)
Now, it'll be easy enough for anyone to argue that the teen is responsible given the typical age of reason argument. Defence might argue that he has social development issues and therefore is emotionally younger. Prosecution may argue that given he researched, developed and then executed his 'attack', the level of premeditation and appropriate amount of time taken to complete would show presumptive responsibility.
In other words, dude. wth?
Yes, the engineers built a cheap system however many years ago (i'd say decades). They didn't have any idea of what they needed to do for security. And for what it's worth, it'd be pretty easy to hack SF's Muni - there are plenty of spots where all you need is a welding torch and a crow bar to cause damage and risk lives.
But the reality is, the kid did something wrong and will find it pretty hard to argue that he's not responsible. Hopefully however, the judge will have the foresight to set a sunset clause on his sentence which will expunge it from his record when he turns into an adult -- or the hope of him ever getting a visa to come and hack out here will be pretty slim. :)