1. He's old chances are he was killed because he isn't useful in the Legion's eyes. Just look at the end slide for The Great Kahns they join willingly and still have their old and sick killed and, have their women turned out.
2. As to the mutants, we never really see the Legion confront any (Super Mutants, Nightkin, Ghouls, ect) so we have no idea how they'd treat them, but if I had to guess they'd attack them on sight.
3. I also want to actually see some citizens before I actually believe they have them. As far as we're aware they are Military, Baby Makers, and Slaves.
1. The Khans elders were wiped out cause the tribe was going to be integrated into the Legion. I don't see why they would care if a citizen is old.
I know, I know, it
does sound flimsy as [censored] who they decide to kill, enslave, integrate, allow citizenship.
2. From wikia: "The Legion openly discriminates against super mutants and ghouls, considering them beasts. However, they will not go out of their way to attack or destroy them, if they do not provoke an attack."
I'm going by this bit from the wikia. Legion has no real quarrel with mutants, they don't want them in the Legion, they most likely don't want them as slaves as ghouls are rotting flesh, nightkins are crazy and regular super mutants aren't too bright all the time either.
So what's left?
Kill them?
They could but they have no reason to do that.
I'll be going off of that wikia excerpt until we get some new lore telling us otherwise about Legion.
But frankly it would be a saving grace of the Legion if mutants, while discriminated, are allowed to live free in their lands.
Legion has so many "negative" points that it could really use another good point.
So hopefully Obsidian don't [censored] [censored] up and make Legion even more evil... <_<
3. In the Vegas ending slider for Edward winning it says he enslaves much of the population and "peacefully lords over the rest". They also guard Primm in on of the ending sliders. Then we have Dale who's free to roam around Legion lands and trade, not to mention they have their own currency.
While we have no 'carved-in-stone' fact that blatantly points out that they have citizens it is hinted several times