From a rehabber's perspective, the 30's-50's are when the some of the good older practices of construction were still in use (ie, "overbuilt"), but the materials were much more consistent than in the late 19th and early 10th century.
I have a coach house that was built in the 40's behind a 1880-built brick home; the latter is very solid, but the brick isn't nearly as strong as the '40's structure. The coach house is rock solid, with little degradation or signs of settling.
In my opinion, it doesn't matter when the building was built as it concerns renovation: you're still using the most-recent, up-to-code solutions in the job. But mid-century buildings are a nice compromise between strength & "character" (which I really think helps sell leases).
Older buildings have a higher possibility of deferred maintenance, of course, but it's really the maintenance or lack thereof, not so much the age, that makes the difference.