This is an interesting question, and there is no definitive answer. In Frindle by Andrew Clements, Mrs. Granger is the fifth-grade teacher in the Westfield School District, and she is one of those teachers. She is passionate about the things that matter to her, but she is strict--very strict. She likes words to be what they are and used as they were intended to be used. She has quite a reputation for being strict, and on his first day in Mrs. Granger's class Nick Allen discovers the rumors are true.
We do not learn exactly how old Mrs. Granger is, so it becomes a matter of speculation, then. We can presume that she began teaching right after she graduated from college, probably at age twenty-two. She has been teaching long enough to gain a reputation, so perhaps five years or so. At the end of the story, ten years later, she is still teaching. She might be as young as her late twenties or as old as her forties or even fifty when the story begins. It is simple and speculative math rather than any real information.
More importantly, it is essential to note that if Mrs. Granger's age were a significant factor in the story, the author would have told us her age. In this novel she is more of an archetype, a certain kind of stereotyped and ageless character. Mrs. Granger is the teacher who is strict and demanding in the moment but serves, long-term, as a motivation and inspiration for her students to achieve great things. The author was asked where he got the inspiration for the Mrs. Granger character, and he said this:
Mrs. Granger is...constructed from bits of many teachers I have known and some I have worked with, again, including myself.
Her actual age matters less than her role as "the bad guy" in the story.
"Every good story," Mrs. Granger writes to Nick, "needs a bad guy, don't you think?"