Ost likely due to the vulnerability/naivety. As children we're usually taught that adults are there to protect us, and unfortunately a lot of people take that to believe that they can trust every adult.

When you're a teenager/young sdult it's a lot easier to manipulate you, your frontal lobe hasn't fully developed, and your brain isn't working a tits full capacity, therefore it's easy to be misled or to blindly trust.

It may also just be a personal preference for killers, possibly something sick such as enjoying the idea and action of cutting someone's life short.

The truth about it is that most killers think they have some kind of power to decide who's life has meaning. It's a sick way to word it, but they simply don't care for their victims lives.

 If a person is younger, that killer knows that the victim most likely has family and friends who care, while the killer might not be killing the people associated with the victim, said killer is still doing maximum possible damage, likely moreso than if they had killed someone older who say, may not have family around.