I just ran across this, via Fred Hembeck's web site.
Linked below is a response letter sent to people who wrote to DC comics circa 1968. It appears to contain answers to several frequently asked questions. Some of these answers I've never seen before.
Here's page 1Here's page 2The referencing page contextSome interesting quotes from it:
In the matter of our characters' ages, we use a good deal of "poetic license." This is a common practice in comics and has kept Little Orphan Annie a "little girl" since 1924. At present, we consider Superman to be about 32, Lois Lane and Lana Lang are a year or two younger. Supergirl was 15 when she came to Earth and is now about 19. Jimmy Olsen is 23. Lucy Lane is 21. Perry White is about 55.
Superboy was 2 years old when he arrived on Earth, and his foster parents, the Kents, were in their 30's. Most of our Superboy stories occur when the Boy of Steel is between 14 and 17; they don't always happen in the exact order in which they are published. At this time, Jonathan Kent is about 50, while Martha Kent is a years or two younger. But they appear younger since a serum from another dimension restored their youth.
I'm a little surprised at the age of the Kents; they seemed older than 30s (Superbaby) and 50 (Superboy).
Whether Supergirl will get her own magazine has not yet been decided. We tried her out by teaming her with Wonder Woman in an issue of THE BRAVE AND THE BOLD, as well as giving her an 80 PAGE GIANT of her own. We may give her a SHOWCASE tryout in the future. If that proves a hit, we'll think seriously about giving her her own book.
This letter apparently was sent out over years and revised; I notice the heading uses an old postal zone code (22) in the address (not used since early 1960s), but the return address uses a standard ZIP code (10022). It appears to be up-to-date with events that happened in 1968, such as the Kents becoming younger, and Chemical King, Shadow Lass, and Timber Wolf being members of the Legion (Adventure 372, Sept 1968 for two of those).