There's a difference between psychosocial 'maturity' (ie not acting like a twat) and neurological maturity (changes to the composition and function of the frontal cortex). I'm not letting him off the hook, this is clearly not something where he couldn't possibly know it's out of order. What I'm saying is that because of his age and maturity in the latter sense, once he had the idea and took it seriously (which is the scary part and needs fixed) he was less able than someone five or ten years older would be to resist the impulse to carry it out.
I'm no saying he's a victim here. What I am saying is that the sentence appears lenient only because it has taken this into account: from a punitive point of view, he was less able to control his actions than an adult would be and diminished responsibility is regularly taken into account; from a public safety point of view, there's an assumption that he will be less likely to reoffend than someone who did this as an adult (because the adult's never going to develop greater capacity for impulse control); and in terms of rehabilitation and psychiatric involvement, as above, there will be percieved to be a greater chance of success in a shorter period of time than otherwise.
It's a short sentence because a longer one would serve no purpose other than institutionalising and criminalising the wee shit.