While I don't think anyone's life is worth more than another, and an old person should get the transplant if the younger person needing it did drugs, drank alcohol, smoked, or is just generally at fault. However, if the decision has to be made between two people who aren't at fault, then the young person should be chosen because young people have more potential than a person who is, let's say 80 years old.
Age should be a factor, because you want the recipient to survive as long as possible with their new organ. In the case of a wild age difference like an 85 year old versus a 12 year old, it's obvious who will get the most use out of the organ. If recipients are closer in age, than other factors such as addictions, lifestyle, and any systemic/chronic conditions should be considered. For example, an otherwise healthy forty year old will probably live a longer and better life with a new organ than a chain-smoking, alcoholic, diabetic thirty year old.
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The whole argument here can be fought for forever, until we are all old. As the fact is, no one lives forever. And in my hopeful opinion no one ever will. But we should definitely put more effort into our future than the present. The younger ones are the ones who will grow, and then the circle of life would continue. However if we out the energy and time into that of an elder, they would die soon after anyway, as their body would die with or without transplant. This should however be countered by lifestyle, as I see quite often in the Pro elders. The lifestyle is extremely important, if we were to have a smoking, drug abusing, 23 year old male, versus an older man suffering from heart failure, the elder would most likely be put on priority. Though ethics may also be taken into consideration, this argument could go on forever.
It might seem obvious to some that a 12 year old should get a transplant over an 85 year old, but what about if they're closer? Does a 25 year old deserve a transplant more than a 50 year old? What about 30 compared to 40? 35 compared to 36? How would we go about deciding who's life is worth more? It is impossible. Any line keep in mind that any line we draw would be completely arbitrary.