Assuming no anti-aging progress, me being 90 implies that I've got 15 years left of life, tops... and most likely 2-4 given that I've just had a transplant. I'd be willing to give that up in order for someone else to live, as long as I can die a decent death. I believe that there are benefits to doing the right thing, both in the here and the hereafter.

It'd be a harder decision if I was 50 and had children who depended on me, and if the 20-year-old didn't have good odds anyway (or had done something incredibly stupid). Then I might be tempted to do make the "selfish" decision if it were mine to make. But not if I am 90 and decrepit.