The simplicity in the scenarios and the ambiguity of the scenario in this type of question is desired not a fault. The goal is to set up a stage for exploration of that nuance and "shades of gray" space just as you started exploring it in your comment. The goal is not to set up the question in some way that makes it so one of the answers or reasonings is right or appropriate. As such the stats only talk to which lever was pulled, if the question could have all of the reasonings laid out beforehand in a single page then it wouldn't have been a very good trolley type question.
As for talking about the reasoning freeform that's why it was posted here! For your particular example I chose the old people because of QALY. 5*<last 10 years of life> is fewer QALY than 50 years of life starting at a young age. There are probably a half dozen other reasonings I could think of and many more I can't and that's exactly what the goal of the question is, not about actually finding a most correct answer via rigid framing.
The trolley situation is just meant to frame the "you have to choose" type decision without starting back and forth "whatabouts" that avoid looking into the actual question like "I simply wouldn't get into such a situation" or similar.
It's impossible to make some scenario 100% of people will find bulletproof so people just use the trolley scenario to convey the concept the question is about the balance of the decision not how the need for the decision came about.