The legendary stage and screen actress, who plays the haughty and formidable matriarch, the dowager Countess of Grantham, also hinted that the hugely popular British period drama may be coming to a close at the end of the sixth season.
"They say this is the last one, and I can't see how it could go on," said Smith in an interview with the Sunday Times, adding, "I mean, I certainly can't keep going. To my knowledge, I must be 110 by now. We're into the late 1920s."
The Emmy and Golden Globe Award-winning period piece, which follows the lives and loves of the Crawley family and its servants in their stately Georgian country house, was set in 1912 when it launched back in September 2010.
It was 1924 when the show's fifth season concluded on PBS in North America earlier this week.
Creator has new project
Julian Fellowes, Downton Abbey's creator and writer, has signed on to a new NBC series called The Gilded Age, which follows the upper crust of New York's society in the 1800s.
A pilot is in the works for this year, and there are rumours that the very hands-on writer will have to drop Downton Abbey to do The Gilded Age full time. But Fellowes has said Downton Abbey's fate is out of his hands.
"I don't own Downton Abbey now. NBC Universal owns Downton Abbey," the 65-year-old Oscar-winning screenwriter told The New York Times.
"I could walk away, but I wouldn't walk away. It's too much my baby," said Fellowes, adding, "It won't go on forever — I'm not a believer in that. But I can't immediately now tell you where the end will be."
Downton Abbey's sixth season goes into production later this year.