While Jaime and Brienne are taking their bath at Harrenhal, and he’s sharing the story of how he became the Kingslayer, he asks himself:
I’d say Brienne’s age is in the 18-20 range when we meet her in A Clash of Kings. She’s past the age of majority, but she is hilariously, adorably naive. Which explains a lot about her going off to serve King Renly! Jaime’s in his 30s, and suffering no lack of cynicism.
And, indeed, Jaime: why ARE you sharing your story with this “ugly absurd child” when you haven’t even told your own siblings?