"Cruel and unusual punishment" is an appeal to emotion.
It's a fact both that children have less life experience and that highly addictive and dangerous substances are difficult to learn from and adapt to. Heroine will addict you in 3-4 uses (not 1 like people say), and after fairly short-term use will put you in a situation where withdrawal can be fatal--not to mention your brain is screaming for it. I know folks who have been through it and gotten out of it as young as 12-14; their anecdotes are always the same: it's only their emotional support network of family and friends that pulled them out, otherwise they'd have never gotten off no matter how bad they'd wanted to.
The same can be said of student loans--18 year olds don't know anything about finances, and hell even most seasoned adults who have trudged all 30 years of a mortgage don't get it. How anyone thinks deferred loans are a good idea is beyond me. Maybe it's because people put too much value on education--more value than the sum total of their life, not to mention a stunted career unless you're in some form of education-driven skilled labor (i.e. medicine, law, higher level engineering/mechanics i.e. rockets and planes). Experience-driven skilled labor (automechanics, IT, management) benefits more from entering the career field early, taking college slowly, taking a broad-base education, and taking minimal and short-term debt.
It's not fallacious to assume that children and young adults are less experienced and more vulnerable. It is in fact a powerful strategy to market wares to the naive and build psychological attachment so that they become a profitable adult market--it's called 'grooming'. Drug dealers would do best to market their wares to young teens and turn them into addicts just like cigarette companies used to. This isn't an appeal to emotion so much as an acknowledgement of a fact.
Likewise, I specified that we're applying certain rules to things we determine to represent a wide social threat. Highly addictive substances are important; substances we're uncomfortable with but that represent low risk are not as important. Executing drug dealers for selling marijuana to kids is an appeal to emotion--especially with arguments about gateway drugs and their future forays into cocaine and prostitution to support their habits. Citing a problem that represents a societal threat to the adult population and tracing it back to the impact of indoctrinating young, mouldable minds is simple strategy.
As for cruel and unusual punishment, we live in a world where a highly immature and uncivilized segment of society has used a huge appeal to emotion to convince people that folks don't really fear pain or death. Execution is not a deterrent to murder, and the infliction of pain (called "torture" even if we're talking about something as banal as caning, which is little more than a short, painful beating) is thought to be a horror. For our trouble, what we do is shove people into prison for eternity instead of executing them; and, when we do execute them, we do so by injecting them with anesthetics so they feel no pain as they die peacefully.
How cruel it would be to take a poor man who steals a loaf of bread, beat him two dozen times, and send him home to his family. He might be sore when he returns to work the next day to barely earn money for food and rent. What we should do, what we do instead now, is send him to jail for 45 days, 60 days, 90 days. When he comes back he will be behind in rent, and his family will be starving, possibly evicted already and on the street; he likely will no longer have a job; but at least we're not so cruel as to drive screams of pain out of him by vicious application of the cane! No, we're much better sentencing him to a life on the streets where his best options are petty theft--or, perhaps, to become a drug dealer and have a chance at affording a home again one day. This is the superior, civil method of dealing with criminals--not that barbaric display of cruelty our ancestors used.
Watch what fallacies you call. You might want to look a little harder to see if perhaps you're standing on that very island.