Here's an oddity that consistently pops up in the Superman family of magazines. I'd guess that whatever age we thought Superman was when we were kids, nowadays we'd put him in his mid-late 20s at the youngest. It doesn't really make a whole lot of sense for him to be much younger than that. For starters, we are always told that Clark is one of the top reporters for the Daily Planet, a major metropolitan newspaper. It is often mentioned that he has won numerous prizes for his reporting, and it is clear that he was graduated from Metropolis University, which means that he could not have started his career much before age 22.
However, there are problems. In the advertisements for Lois Lane #1, they gave her age:
Okay, so let's assume that maybe Clark/Superman is a little older than Lois. This makes some sense; it explains why he seems less interested in her than she is in him. It also might explain her frequently zany antics and risk-taking behavior.
But even this solution has its drawbacks. For starters, it was established in the early Superman stories that Lois was a reporter at the Planet before Clark arrived there. And indeed, there are several Silver Age stories that confirm this. For example, in Superman #133, there's a story that tells how Clark got his job at the Planet, including this scene:
So maybe Clark earned his chops at some smaller newspaper, before going to work in the big city? That's arguable, but the problem is that there are many stories which establish that Lois and he are the same age (or at least very close). For example, in Superboy #90, there's a story about how Lana was able to look into the future and observe the "romance" between Superman and Lois Lane. So she visits nearby Pittsdale (Lois' hometown) and attempts to sabotage prevent her from becoming a reporter:
But if Lana (who is the same age as Clark) is a high-schooler at the same time as Lois is, then Clark cannot be more than a couple of years older than Lois. And there are other stories which establish that they are actually the same age. For example, consider this famed story, from Adventure #128:
In addition, there is another story where Clark is shown as reasonably the same age as this young woman:
And yet, Clark is shown as meeting up with her in Metropolis years later, after he and Lois are reporters, and she is not yet 21!
So the conclusion seems obvious. Superman is 21 or 22 years old in the stories we read as kids. But even this causes problems (beyond the question of how he got a prestigious job at such a young age). Consider this letter to the editor from Superboy #68 (October 1958):
Indeed, many of the early letters columns in Superboy complained of such supposed anachronisms. But think about it for a second. World War II had been over for 13 years by then. If Superboy was in high school in the Superboy stories, then he must be at least 13 years old in those tales. Plus 13 years would make him 26+ in the Superman stories. Or looking at it the other way, if he was 22 in the Superman stories, then that would indicate that the Superboy tales were taking place around 1949 or 1950, when the television antennas would not be an anachronism.
But apparently chastened by this reader and others like him, Weisinger was more careful to make sure that Superboy didn't encounter anything too modern. Consider this scene from Adventure #253:
That dates the lecture to no later than mid-1945, or about 13 years before the 1958 publication date. But in the story, Robin the Boy Wonder has come back in time to visit Superboy (as discussed here). Robin appears to be about the same age as Clark Kent in the story. But Robin is often shown driving the Batmobile in Batman stories of the time, which means he must be 15 or 16 at the youngest, which again would place the contemporary Superman at no less than 28 years old.
The conclusion? Superman is somewhere between 21 and 28 years old, depending on the needs of the particular story.