Admittedly fairly obvious choices, but anyway:
1. Miss Marple -- who may or may not have cracked 70 at the beginning of the series (The Murder at the Vicarage, 1930) but is an elderly lady even then and must have been over 90 by the time the last book about her was published, some 46 years later (Sleeping Murder).
2. Allan Karlsson -- the eponymous protagonist of The Hundred-Year Old Man Who Climbed out of the Window and Disappeared.
3. Little Nell's Grandfather in The Old Curiosity Shop.
Sherlock Holmes -- who has retired and is keeping bees in the South Downs in The Final Solution, which is set in 1944.