The seer was already very old when the series began, but now all the characters are much older and he still looks the same.
How is that possible? Is he some supernatural being? Or is it someone else just looking very similar?
The outward marks of aging may have come earlier in the past due to poor understanding nutrition among other factors, so possibly the seers is younger than appearances suggest. Thus it's possible that not only the seer but the main characters are younger than we expect.
There is also a theory that it's fairly easy to live into 80 in the modern era, but if you got anywhere near that age in the past, you probably had exceptionally good genes. (i.e. there no modern medicine to give you that bypass operation, etc., to extend your life. )
The presumption is that if the seer could make it to old age in the first place, making it to extreme old age is not unlikely.