Scene between Judi Dench and Maggie Smith:
Judi: I don't know why I tell you anything.
Maggie: Because I'm older and wiser.
Judi: 19 days older.
Actually, Dench is 19 days older than Smith, born on Dec 9, 1934, while Smith was born on Dec 28, same year.
I couldn't help but notice that either, even though, they were only playing characters, so it isn't really important what their birth order was. And 19 days is hardly noticeable. But the truth is, it's not the first time Judi Dench was cast as younger than her – she was also cast younger than Maggie in LADIES IN LAVENDER.
I think it has to do with how Judi carries herself. When she's not busy playing a strong character like in the Bond films, she typically plays a character with a zest for life, and a childlike outlook. Plus, it's kind of hard to ignore her Elvish, pixie like face.
Please excuse typos/funny wording; I use speech-recognition that doesn't always recognize!