Montag is thirty. After he first meets Clarisse, they talk briefly, and she asks him how long he has been a fireman. Montag's response is "Since I was twenty, ten years ago." So, at the beginning of the book he had been burning books for a full ten years-a significant amount of time, especially considering how in the next sections he turns against it completely. I also provided links below to a summary of the book, along with other information. Good luck!
As the above answer stated, Montag is 30 years old. He tells Clarisse he's been a fireman for 10 years, since he was 20. Thematically, this age is significant. While 30 might seem old for students reading Fahrenheit 451, 30 years old is a time of transition with the true end of youth and the entrance into adulthood. This is when people truly start figuring out who they want to be. This is true of Montag.
When Montag meets Clarisse, she throws his life into upheaval. She asks him many questions that challenge everything he has accepted in his youth. She asks him if he ever notices things like the man on the moon and dew on the morning grass and introduces the ideas of talking with others just to talk. But the big question Clarisse asks Montag is, "Are you happy?"
Montag ponders this question the rest of this chapter, but the answer is immediately obvious when Montag enters his home and thinks about his wife. The narrator describes the features of Montag's home as a "cold-marbled room of a masouleum" and a "tomb-world." Then he describes how how Montag imagines his wife "stretched on the bed, uncovered and cold, like a body displayed on the lid of a tomb."
While 30 is a transitory part of life, the questions Clarisse asks Montag continue to haunt him. She inspires him to throw away all he has built for himself and transition away from a life of obedience to a life of rebellion.