Daniel could be about 60 years old. Even now, when people survive a lot longer generally, 60 is old enough to be considered "old" by plenty of people. He has a mental disturbance, but even if it was referred to as senility associated with age (and I'm not sure they did), that doesn't necessarily mean that that's what it really was. Maybe it's after-effects of traumas, from the events of 1795, or what happened with his wife. Or maybe it's medical, but not age-related. Or it IS senility/Alzheimer's, which has been known to affect people in their 50s.
Daniel doesn't appear to be infirm from old age. They keep him locked up, and there's nothing for him to do in that room but stay in bed, really. Once he's out, he walks around well enough.