In the book Fahrenheit 451 how old is Clarisse?

Clarisse is seventeen years old.

Clarisse is Montag’s neighbor.  She knows that he is a fireman and seeks him out one night.  He is surprised that a teenager would be out so late by herself, so he asks her how old she is.

"I'm seventeen and I'm crazy. My uncle says the two always go together. When people ask your age, he said, always say seventeen and insane. Isn't this a nice time of night to walk? I like to smell things and look at things, and sometimes stay up all night, walking, and watch the sun rise." (Part I)

Clarisse is an unusual girl from an unusual family.  While most people in Montag’s community like to drive fast and spend all of their time watching television, Clarisse is a slow down and smell the roses type of girl.  She and Montag have a good talk.  She asks him if he is happy, and it never occurred to him to ask.

Montag sees Clarisse often after that, and he comes to get used to talking to her.  She is so different from everyone else he knows, because they are all distant and shallow.  One day he asks her why he feels like he has known her for a long time. 

"Because I like you," she said, "and I don't want anything from you. And because we know each other."

"You make me feel very old and very much like a father."

"Now you explain," she said, "why you haven't any daughters like me, if you love children so much?" (Part I)

He doesn't have children because his wife Mildred is little more than a statue.  All she does is watch television and listen to the radio.  They barely talk to each other.  She is not interested in children.

One day Clarisse is just no longer there.  Mildred tells Montag that she was killed when a car ran her down.  He is shocked that Mildred would not want to mention that.  It is no big deal to her, just a neighbor being killed.

Montag asks Beatty about it, and he says Clarisse is better off dead.  She and her entire family were trouble.  They were independent thinkers, and you can’t have that.  This confuses Montag and he questions it, which fits with his new worldview.  His society does more than ban books—it bans independent thought.

Clarisse is a catalyst for Montag.  She breaks him out of his shell of indifference, and makes him question himself and his actions.  Although she is only in his life for a short time, she makes a big impression on him.  In many ways, his pursuit of knowledge is a tribute to her memory.