Writing someone who is old, but not impaired by decay is very, very difficult, due to lack of examples, but I think this might be less of a leap than it seems.

George Bernard Shaw wrote some in his play "Back To Methuselah". It is long; search for "Lutestring" (the name of one of the characters) and read forwards from there. Context: Mrs Lutestring and The Archbishop are, covertly, over 250 years old, and their secret (previously not even known to each other) has just come out. The others in the scene are of ordinary ages and, as far as they know, short-lived. Then search onward for the subtitle of act 5, "As Far as Thought can Reach", set in the year 31,920 A.D. (Shaw's speculative mechanism for life extension can be ignored.)

And of course there is Lazarus Long.