Reliable health and medical practices hadn’t even been established, so there weren’t very many ways to stay youthful-looking which could be counted on (if such a thing was even on the radar of most people).
Think about how spotty Claire and Jamie should be by now, with all of their time outdoors.
I mean, they didn’t even have sunscreen back then, you guys.
Not only did they not have gyms, and had to rely on hard, physical labor to keep them fit (which I’m sure worked pretty well, while not giving anyone a body which would mimic that of a modern fitness model), they were also without some things most of us would consider really basic.