Here's how I would assign the official ages if given control by DC:
Superman would have begun his career at age 30 (per the Richard Donner movie, as well as reinforcing the Christological connection to the start of Jesus' ministry), and would perpetually be portrayed in the present as being 38 (established, but not quite pushing middle age).
Lois Lane would have her role reversed with Jimmy Olsen slightly to make him closer to Clark's contemporary and her having been the college intern at the Daily Planet when they first met. Thus Lois would have been 19 when Superman first came to Metropolis and Jimmy 21, with them being 27 and 29 in the present, respectively. After all, being Superman means he could have any* woman he want; what reason would he have to rob the grave?
Batman would be approximately the same age as Superman, and both would have begun their superhero careers independently of one another at the same time.
Wonder Woman would be of indeterminate age, having lived on Thermyscira for decades or even centuries before coming to the world of man, having ceased to age the moment she became post-pubescent.
Most other male members of the Justice League, such as Hal Jordan, Barry Allen, and Aquaman, would be in their mid-thirties (for these specific examples, 36, 35, and 32, respectively), having begun their careers just slightly after Superman's public debut.
Adult female characters would have had shorter careers, only coming onto the superhero scene in the last year or so in comic-book-time, so as to keep them in their early twenties. Mera, for example, would be 24 in the present, and Power Girl and Huntress both 23.
Nearly all teen character, such as Dick Grayson, Donna Troy, Wally West, Roy Harper, Cyborg, Starfire, and Beast Boy, would be 19 in the present. Supergirl would be an exception at 16.