First, comparing the judgment of an 8 year old to that of a teenager doesn't wash. Even before factoring in the enormous changes in cognitive thinking that take place after age 8, the levels of experience are so dramatically different as to render comparisons worthless. Simply put, 8 year olds are not teenagers.
Second, comparing a teenager committing a criminal act (attempting to buy heroin is illegal) to an 8 year old minding her own business is also meaningless. The teenager is going out of his way to put himself in harm's way--committing crimes is dangerous. The 8 year old girl was not seeking out a dangerous situation.
Finally, we come to those cognitive differences. An 8 year old wouldn't recognize the danger of an oncoming car until it smacks her; a teenager generally would. Abstract reasoning pretty much doesn't exist in an 8 year old--that's why rules for kids that young about "no talking to strangers" generally fail: they are cognitively unable to understand the term "stranger".
Any one of these defects alone renders your logic fatally flawed. That you cling to it in the face of decided and deserved condemnation is morally suspect.