The difficulty with minimum sentencing is that it could conflict with the Charter of Rights as being cruel and unusual punishment. It will only be saved if it qualifies under section 1 of the charter. That being said, this would explain why you have minimum sentences for murder that are constitutional—because section 1 saves them.
You might recall that several years ago the Supreme Court reviewed a case of smuggling narcotics into Canada, at which time there was a seven-year minimum sentence. That minimum sentence was struck down on the basis that it was unconstitutional. So, leaving aside the merits of a minimum sentence, you have the hurdle of whether or not the Charter of Rights is offended by virtue of it being cruel and unusual.
You can envisage a 16-year-old, in the particular statute that we're talking about, varying in comprehension, varying in a number of ways for which it would be desirable for the court to have some discretion.
I don't know that I can say much more on the problems with minimum sentencing, the hurdle that has to be overcome.