issue of Blanche's age arises a number of times in the play, usually raised by Blanche herself. She raises the issue in scene 3, when she lies about her age when she first meets Harold Mitchell, known as Mitch.

MITCH. You are Stella's sister, are you not?

BLANCHE. Yes, Stella is my precious little sister. I call her little in spite of the fact she's somewhat older than I. Just slightly. Less than a year. (Scene 3)

A little later in the scene, Blanche is somewhat self-deprecating about her age, perhaps fishing for a compliment from Mitch, which she gets.

MITCH. You're not—?

BLANCHE. Married? No, no. I'm an old maid schoolteacher!

MITCH. You may teach school but you're certainly not an old maid. (Scene 3)

In scene 5, Blanche and Stella are talking about Mitch, who's coming to see Blanche later in the evening, and Blanche gives the audience a sense of the life she's lived and the effect it's has on her face.

BLANCHE. ... Mitch—Mitch is coming at seven. I guess I am just feeling nervous about our relations.... Of course he—he doesn't know—I mean I haven't informed him—of my real age!

STELLA. Why are you sensitive about your age?

BLANCHE. Because of hard knocks my vanity's been given. (Scene 5)

In Scene 6, the issue of Blanche's age comes up again with Mitch, whose mother has asked Mitch how old Blanche is.

MITCH. Can I ask you a question?

BLANCHE. Yes. What?

MITCH. How old are you?

(She makes a nervous gesture.)

BLANCHE. Why do you want to know?

MITCH. I talked to my mother about you and she said, "How old is Blanche?" And I wasn't able to tell her. (Scene 6)

Blanche manages to avoid answering the question and changes the subject. Mitch has yet to see Blanche's face in full light, and any answer that Blanche might give to Mitch's question about her age, particularly with regard to Blanche's reference in scene 3 to Stella as being a year older than Blanche, will fall into serious question when Mitch actually sees Blanche's face.

In scene 8, after what Williams notes was a "dismal birthday supper" because Mitch failed to show up for Blanche's birthday party, Stella lights the candles on the cake, but the party is interrupted by a phone call for Stanley. At the end of the phone call, Stanley takes an envelope from his pocket and gives it to Blanche.

STANLEY. (with false amiability) Sister Blanche, I've got a little birthday remembrance for you.

BLANCHE. Oh, have you, Stanley? I wasn't expecting any, I—I don't know why Stella wants to observe my birthday! I'd much rather forget it—when you—reach twenty-seven! Well—age is a subject that you'd prefer to—ignore!

STANLEY. Twenty-seven? (Scene 8)

When Mitch finally sees Blanche's face clearly in scene 9, he isn't as concerned about her age as he is about the fact that Blanche has repeatedly lied to him about her life.

[Mitch crosses to the switch. He turns the light on and stares at her. She cries out and covers her face. He turns the light off again.]

MITCH. (slowly and bitterly) I don't mind you being older than what I thought. But all the rest of it—Christ! That pitch about your ideals being so old-fashioned and all the malarkey that you've dished out all summer. Oh, I knew you weren't sixteen any more. But I was a fool enough to believe you was straight. (Scene 9)

Post #7 by sagate is correct. On page 4 Stella is described as "about twenty-five" in the stage directions, and then on the next page the stage directions describe Blanche as "about five years older than Stella." Being thirty years of age does not seem terribly old. Blanche also brags about her trim figure. Her biggest problem is with her nervous condition, which causes her to do a lot of drinking. Her anxiety and drinking may have caused her to look older than she actually is. She is chronically worried about her appearance. She tries to get compliments from Stella, from Mitch, and even from Stanley.

I agree with the above posts that label Blanche as being in her thirties.  During the play, Stella is pregnant, so one would assume that she is in her twenties (given the time period) and although Blanche is older, it does not seem likely that she would be more than ten years older than her sister. 

Because she hides so from the light, Blanche must be quite a bit older than her sister, and she does act somewhat authoritatively toward Stella.  So, she can be in her late thirties.  At the time in which A Streetcar Name Desire is set, this age would be considered past prime for childbearing and even for marriage for most men.  Thus, Blanche must dissemble.  And, since she has once been married, putting Blanche in her thirties seems plausible.

I think that Blanche would be in her thirties.  I cannot help but see Vivien Leigh as a great Blanche.  When you bring up Rachel Weisz as the next Blanche, it got me thinking to what characteristics are needed.  You need someone who is able to strike that balance of being able to play multiple personas, sometimes at once, and be able to pull it off in a convincingly.  At the same time, you need to have an undercurrent that she's one or two steps from unravelling.  She does talk a lot and you need someone charismatic to be able to accomplish this, but I also think that you need someone who can communicate real sadness and a real sense of being utterly human.  It's really hard to be able to to find someone who can take all of these and put them in a form that makes complete Blanche's characterization.  Leigh was able to pull off Blanche and I think that's why so few others dare to go near it.

I've always thought of Blanche as being in her 40's. The fact that she can still be attractive and flirts with men indicates that she still has a very strong sexual desire. She still tries to get Mitch and the young man who calls at the door, so she must see herself as still young enough to be a "catch". She has also had time to fail at marriage, to get a bad reputation, to be fired from her teaching job for having a relationship with a student and to lose her childhood home. So, she might be in her late 30's or early 40's. She's done too much to be in her 20's. And I think 50 is too old, especially since the play takes place in the 1930's. At that time, 50 was almost the end of one's expected lifespan.