Astounding Stories of Super-Science began publication in January 1930; changed its name to Astounding Stories in February 1931, and again to Astounding Science Fiction in March 1938; it became Analog in October 1960, at which point the tone and content of the old magazine was largely lost.

I say this to preface things: Astounding was long-runner by pulp standards, and kids that grew up with it in the 1930s could see their own kids read it in the 40s and 50s, and maybe their grandkids picked up the last couple of issues on the stands in 1960. In all that time, there was never anything like a comprehensive demographic poll of the readers. But we can say a few things, and infer a few others.

The early competition for Astounding were Amazing Stories (1926-), Wonder Stories (1929-1955), and in a small way Weird Tales (1923-1954); the audience for science fiction pulp magazines was small, and most of them were buying the same magazines. So to an extent, we can look at the letter pages of the four pulps and get an idea of the demographics: mostly male, although not exclusively; mostly young, when age can be gauged at all; professional scientists and engineers are rare, although arguments about science are more common in the reader's pages of the science fiction pulps. Sam Moskowitz in The Immortal Storm: A History of Science Fiction Fandom noted:

Love of science fiction was the basic bond that united these fans. Yet discussions in The Comet [an early fanzine] were a far cry to discussions of fiction--articles such as "The Psychology of Anger," "Chemistry and the Atomic Theory," "Recent Advancements in Television," "What Can Be Observed with a Small Telescope" and "Psychoanalysis" abounded. As time passed, however, the non-scientific note increased in volume somewhat. Articles based on science fiction stories appeared occasionally. Professionally known authors such as P. Schuyler Miller and A. W. Bernal contributed fiction. Accurate information on German rocketry was printed under the name of Willy Ley. Such luminaries as Miles J. Breuer, Jack Williamson, R. F. Starzl and Lilith Lorraine were also represented.

Some of these folks were scientists; P. Schuyler Miller was only 18 in 1930, but earned an M.S. in chemistry and worked as a chemist and later a technical writer for General Electric and the Fisher Scientific Company, and he remained active in fandom for decades.

Such examples of youthful fans busily engaged in fandom and publishing was very typical during the 1930s and into the 50s; Charles D. Hornig (who would go on to become the editor of Wonder Stories) was only 17 in 1933 when he first published The Fantasy Fan. The crossover between fandom and "professional" science fiction writer for the pulps was almost constant; Lilith Lorraine had appeared in both Astounding Stories and Wonder Stories Quarterly before appearing in The Comet, and would continue to be involved with fandom for decades. Julius Schwarz and Mort Weisinger, editors of the Science Fiction Digest, would become literary agents; Schwarz would go on to enjoy a long career at a little pulp off-shoot named DC comics.

It should be noted that science-fiction fandom at this point was widespread; letters came into Astounding from all over the country and the English-speaking world. New York, however, was an editorial hub of several pulp publishers (and later comic books), so there is a locus of activity there - Julius Schwarz could drop in at the offices of Astounding in New York, and sometimes did. The large density of fans in places like Chicago and New York is why several early fan-conventions happened there...but you also get fans from all over the country, from small towns in Wisconsin like August Derleth and Virginia "Nanek" Anderson, military brats like R. H. Barlow, etc.

There were generational gaps, in both experience and education. H. P. Lovecraft never finished highschool, though his erudition and lifelong interest in science had many mistake him as a college graduate; Clark Ashton Smith was largely an autodidact; Robert E. Howard's college experience was limited to courses at the Howard Payne business school, for bookkeeping; C. L. Moore had to drop out of college to work through the Great Depression, and didn't complete her degree until the 1950s. Others had considerable education - in or out of school. Arthur J. Burks and E. Hoffmann Price both served in the military, as did most of their generation, and Price had graduated West Point; Dr. David H. Keller was a medical doctor, Seabury Quinn was a trained lawyer, Rev. Henry S. Whitehead was an Episcopalian minister. Pulps like Adventure specifically sought out writers who had backgrounds in the fiction they wrote, and Astounding attempted to attract the same caliber of writer in his letters column "Brass Tacks" in the late '30s, with folks like Arthur McCann, a Harvard-educated endocrinologist--although this was, in fact, a pseudonym for editor John W. Campbell, who sought to give his magazine literary or scientific laurels.

Yet most of the young fans were in or finishing high school, and the price was set to match. In time, as the field broadened and pulp magazines specialized, the audience for the magazines began to shift apart. As Alec Nevala-Lee put it in her excellent Astounding: John W. Campbell, Isaac Asimov, L. Ron Hubbard, and the Golden Age of Science Fiction (77):

By the late thirties, each title had evolved to fill a niche--Amazing for juveniles, Wonder stories for teens, Astounding for adults. The first two served as essential bridges to the third, and all three had legitimate readerships.

While there were still adults that read Amazing & Wonder Stories and teens that read Astounding, in general, this is an accurate assessment: Astounding began offering fiction geared more toward adults, with more complicated plots and situations. Julius Schwartz, acting as a literary agent at that point, began advising clients to be more careful about their science (ibid, 79).

A word has to be said about Campbell here, who was editor of Astounding from 1937 through its transition into Analog to his death in 1971. Campbell was racist, antisemitic, homophobic, and sexist; although this was at a time when such traits were common in the United States, they also directly affected his judgment as an editor. Catherine Tarrant, who was effectively assistant editor of the magazine for decades, was only ever referred to as his "secretary." Campbell did publish women and Jewish writers in Astounding - notably Isaac Asimov, but he also downplayed their part in the field, especially in early science fiction, which may partially explain why the latter decades of Astounding seem even more predominantly male and white. To quote Nevala-Lee again (364):

Campbell argued elsewhere that he had no idea what a writer's race might be when he read a submission: "If Negro authors are extremely few--it's solely because extremely few Negroes both wish to, and can, write in open competition." He often touted the high sales of the magazine in black neighborhoods, which he attributed to its policy of "minimizing race problems," but it never occurred to him that the dearth of minority writers might be caused by the lack of characters who looked like them, or that he had any ability or obligation to address the situation as an editor.

Which, if it doesn't give you any hard numbers, I hope gives you at least an idea of the nature and breadth of science fiction fandom, at least during the period of the 1930s. They covered the spectrum of race, age, gender, education, and means--although they tended to skew white, male, and older as the 1930s wore on, and by the time Astounding became Analog the magazine was largely intended for a more adult, more educated audience.