13 years of age is not "an excuse".
She should face the same penalty as any other killer:
Death.
Consider that at 14, there were several of us who had joined the local Fire Department, who worked our summers doing heavy construction and farm labor, and stayed out of trouble...if we could be responsible for our good actions and any misbehaviour, no matter how trivial, certainly someone who acted as a premeditated predator for the purpose of taking a life can be held responsible for doing so.
This 'kid' researched how to pick his victim, how to kill his victim, in a cold blooded premeditated act. Then bragged about the killing online.
He will likely kill again, given the chance.
Only he will likely learn from his mistakes and cover his tracks next time.
The world doesn't need budding serial killers.