Kevin was only six and in the first grade and was made much over by everyone. […] Claudia also thought that he was terribly babied and impossibly spoiled. (1.13)
There's a lot of talk about age in From The Mixed-Up Files, and we see that when Claudia talks about her little brother Kevin, who is terribly immature for his age. We mean, he's only six, but seriously Kevin… get your act together.
Saturday seemed a good day for housekeeping chores. There would be no school groups for them to join. (5.2)
Even though Claudia and Jamie are just kids (after all, they fit in with those school groups just fine), when they run away, they have to take on some pretty adult responsibilities. Like laundry.
"Well, the summer before last, I had just finished the second half of first grade."
"So boloney! It was all I could do to sound out the name of Dick and Jane's dog." (5.49-51)
Claudia needs a reminder that people change as they grow older. Jamie wasn't always as financially sound as he is today, and he certainly wasn't smart enough to read a whole book about Michelangelo.
"I'm glad you asked about homesickness, Jamie. Somehow, I feel older now. But of course, that's mostly because I've been the oldest child forever. And I'm extremely well adjusted." (5.125)
There's a sense of growing up with the whole museum escapade; Claudia has always been the oldest child, but with their escape from home, she feels like she's become more adult. The well-adjusted thing though… we'll have to wait and see about that.
Ages ago, in time well outside the mastaba, they had heard these names—in a classroom, on a school bus […] (7.35)
Talk about dramatic! Even though it's only been a few days since they were on the same bus, Claudia and Jamie feel light years older than the kids in the museum. They are so above it all now that they've lived through (or around) centuries of knowledge.
Claudia was prepared to be the discoverer of great truths, Greenwich's own heroine of the statue- and only twelve years old. (8.32)
Claudia and Jamie are pretty aware of their ages, but they don't let it hold them back. Just because they're young and regarded as small children doesn't mean they can't be heroes, right? It just means that they can be prodigy heroes.
"Big deal," Claudia sobbed. "For all they know, we are grown-ups." (8.43)
When the news comes back that the museum officials already know about the stonemason's mark, Claudia is devastated, even when Jamie tells her that they're treating them like adults. It doesn't matter to Claudia, because they're failing to recognize that they're actually really, really smart kids.
"Yes," Claudia said. "I've been the oldest child since before you were born." (8.95)
Well, there's a duh moment. Of course Claudia's been the oldest child forever. Does she think that they grow at different rates?
It's not a bad face […] These things happen when people get older. And I am getting just that. (9.52)
Ah, the ravages of time and age. Poor Mrs. Frankweiler is caught off guard when Jamie mentions her face and how it's not that scary. There's nothing comforting about being called not that anything…
"Claudia," I said patiently. "When one is eighty-two years old, one doesn't have to learn one new thing every day, and one knows that some things are impossible." (9.254)
Claudia just got told what was up, which is fitting since Mrs. Frankweiler's 82 years on earth totally trumps Claudia's meager 12. Claudia may think that she's the smartest all the time, but Mrs. Frankweiler can give her a run for her money.