In general I agree with you (though I won't comment about this case, as we know so few details). I read a study several years ago whose argument was that the brain doesn't fully mature until the early twenties. Though most teenagers happily are not murderers, most of them aren't all that great at forward thinking, and their moral sense almost always remains somewhat undeveloped. I would not be comfortable with releasing a juvenile who committed murder at age 21 (if that's indeed what would happen - see below), but your suggestion of a middle way between that alleged outcome and sentencing as an adult seems worthy of exploration. 

All that being said, I can barely look at Kiaya's pictures. This is just so sad.