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 think of Blanche as being in her early thirties. The play is set in the 1940's, so a woman of Blanche's that age would be considered past her prime in terms of looks and marriageability. My analysis differs from the previous poster's in terms of Blanche's self-image. While Blanche takes great care with her appearance, she does not see herself as a "catch". She will not see Mitch during the daytime, and will not let him see her in direct light. She freely admits that she must deceive men in order to attract them. Her first marriage happened when she was 16, so fifteen years or so would be plenty of time to experience the things listed above.
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.
In the stage directions when we first see Stella it says she is around 25, and in Blanches introduction it says she is about 5 years older than her sister.