Erin is/was legally an adult. Soldiers dying for their country at that same age although numerically teenagers, would certainly consider themselves men in every sense of the word - as would I. My own mother had me by that age and was managing her own household. Erin could have been enjoying her youth "unencumbered" as you say, but saying she should have been is going a bit far. In researching my family tree, 19 year olds were often well-established in their households, with multiple children. I find it puzzling that, today, people are so quick to consider a 19 year old as more or less a child.
Don't get me wrong. I certainly hope she is found and, if her death was not accidental, I fervently hope the killer is brought to justice. I just think we should pay Erin the respect of considering her as the adult she was.