For a moment in time, the octogenarian and the 3 month old are both in a competition for the same resource, if they both have the same chance of getting it, and the octogenarian happens to get it, the outcome is still fair within this context. It could just as easily have gone to the 3 month old.
It is not fair to count the octogenarian's past good fortune against them in what should be a fair contest. But we do it, because when handing out organs we don't care about fairness, we care about viable lifetime remaining.
When searching for fairness a lot of people seem to believe that good luck in the past means it should be harder for you to have good luck in the future. Ironically, this is very unfair.