I agree that drunk/drug driving should be punished very severely but it's a completely different type of crime to this.
What makes you think she was "very mature" though? To be honest, from the messages that were shown in court, she sounded just the opposite, i'd go as far to say she came across as quite childlike. So whilst I accept some 15 year olds are quite mature, I don't think that's true in this case. Not only that, this wasn't a one off mistake. What Adam Johnson did was planned, manipulative, abusive and took place over an extended period of time.
If he'd met a mature 15 year old on an night out, kissed and got up to something with her in the toilets or whatever we would not be looking at anything like the kind of punishment he's facing now. The grooming element to this is seriously troubling and he will punished as such.