This question is nigh impossible to answer, because of course actors age all the time. As healthcare improves and average lifespans increase, actors will likely continue working to older ages.
However, if we consider only actors who have won a major award, let's say an Academy Award (an Oscar), then the oldest actors to date are as follows:
- Henry Fonda (76 years, 317 days), won Best Actor in a Leading Role for On Golden Pond in 1981
- Jessica Tandy (80 years, 292 days), won Best Actress in a Leading Role for Driving Miss Daisy in 1988
- Christopher Plummer (82 years, 75 days), won Best Actor in a Supporting Role for Beginners in 2011
- Peggy Ashcroft (77 years, 93 days), won Best Actress in a Supporting Role for A Passage to India in 1984
For select others, Eli Wallach acted as recently as 2010 at the age of 94. John Gielgud's last acting credit, in 2000 the year of his death, was at the age of 96.
Here is a list of the 7 oldest currently working actors, as of April 2012.