Severus was both a survivor and older -- by some ten years in comparison to Regulus. Snape suspected that even from a young age, he had looked tired and a bit downtrodden (or, at least, that was a small fraction of what he'd been made fun of for back then) and that probably hadn't changed much; his face was simply sharper now without any last vestiges of childhood there.
And Regulus looked -- careful. Even now. This was something Snape was getting used to. Everyone he'd met thus far from his own world had that going for them -- something careful in the way they cast their glances and the way they choose their words before speaking. Or, perhaps, it was just that way around him. It would make sense.
"I doubt something that happened over two hundred years ago will give you the understanding you're looking for," he pointed out, because Snape rather thought that people so quickly forgot the ugliness of war and what it meant. "I've been here less than two weeks."