Probably Jones's injections.
Cole points this out two episodes later (1x13 Arms of Mine):
Cole: That's Ramse
Cassandra: That's him?
Cole: Yeah. He looks exactly the same. I don't know how that's possible.
Cassandra: Maybe it's Jones's injections. You said... said they make it so time moves around you. Maybe they have an effect on cellular aging.
Following this, there's seemingly confirmation in episode 2x01 Year of the Monkey:
Cole: At least you've got a dose of Jones's injections in you -- it'll help you heal faster and age slower... mostly.
There are more detailed mentions of his aging in episode 2x04 Emergence, which seem to support what Cole said in the above quote:
Nurse: Well, Mr. Ramse, you're one lucky man. Not many people get hit by a car and live to tell the tale. You have osteoporosis in your arm, which is kind of strange for someone so young. And you heal fast. Like... unusually fast.
Gale: Doctors here have a lot of questions about his medical conditions. Like why, uh, parts of his body seem to have aged unevenly.
However, the official recap for episode 1x11 Shonin has this to say:
You know what's interesting about Ramse? He doesn't seem to age, which could be some time travel thing or he just has good genes.
and we later find out that the reason for Olivia's slowed aging is in fact her genes, although she was genetically engineered via time loop paradox -- so the latter is a feasible (albeit very unlikely) possibility.