But what makes eighty special? Would you say the same thing about someone who was younger? "If you're forty, I'd rather feed the four year old"? If not, then why not?

The only objective metric I can think of is that today, 80 is usually not an age at which a person can do much. But if we have technology that lets you live much longer, and people at 80 would be physically like, say, 20-year-old today, that goes away.

And, of course, the number of young people "in the pipeline" can be regulated by controlling birth rates.