So 16 years is the magic point (for Danes? for [Western] Europeans? for all human beings?) beyond which imprisoning people has no impact on crime rates. Do you have any evidence that 1) that's actually the case for Denmark or anywhere else, and 2) that's actually the reason the Danes and others have set that limit?
That's a strange use of the word "secular", for which the Oxford dictionary that Google uses lists as "not connected with religious or spiritual matters". What's particularly religious or spiritual about the idea that the justice system ought to be about meting justice?
It's a very religious idea that "the justice system ought to be about meting justice", particularly a Protestant / Christian idea. The very phrase "meting out justice" is a biblical one.
Secular societies are more interested in the outcomes than the moral righteousness of the system.
Not all secular societies or people are consequentalists. Some are at least as interested in the moral righteousness of a system -- in its means, in its processes, in what it does on the way to its ends -- as they are in what that system accomplishes. There are secular humanist deontologists and virtue ethicists.
That's a strange use of the word "secular", for which the Oxford dictionary that Google uses lists as "not connected with religious or spiritual matters". What's particularly religious or spiritual about the idea that the justice system ought to be about meting justice?