It is true that alot of "first offenders" are merely just 'first time caught'. Now it seems no one ever stepped forward and said anything about middle aged David Westerfield and some prior activities, so perhaps there really are 'first offenders' who are true first offenders, though I would expect that to be rare.

I'm sure its a behavior pattern/ pesonality disorder influence on crime. Ofcourse if you are going to look at criminals and ages, its hard to account for variation across the dataset. And its even harder, in this particular case, to know for sure whether you should be looking up kidnappers, child murderers, child rapists who merely get rid of witnesses, etc.

I think someone like David Westerfield who probably just disposed of Danielle is different than someone who actively plans on and enjoys the killing. David Westerfield wanted a young girl and didn't want her to live to testify against him. However the killer of JonBenet wanted to enjoy the killing a young girl and may not have had any other really major goals in mind.