It does happen. Adults will use young offenders because young offenders will not be punished as severely as the adults.
The issue of deterrence is complex. The deterrent factor primarily is getting caught, not so much the sentence itself. Some research shows that the average person is afraid of being caught by the police, so in fact more police in the community will have a positive impact on the level of crime. Many people don't even know what the sentence is that's associated with a particular crime; it's just the getting caught part.
One positive thing to be said about mandatory minimum sentences is that you will be heightening the awareness of punishment in the community during this discussion. You'll be talking about more serious sentences, and that will filter out to the community in general.