(This isn't really fg specific, but the frame inquestion will be an fg, and fg/ss-folk often ride older frames --and therefore might know the answer to this question or be interested in it.)

Assuming it has been well cared for and that the geometries are the same, is there any appreciable difference between a lugged 531 frame from 1965 and a newer frame of the same materials? I guess what my question amounts to is "barring abuse, do frames (of this sort) deteriorate with age?" I'm inclined to think that they do, but that the rate of deterioration is such that 40 years isn't nearly long enough for an appreciable difference to be noticed. But I don't have anything to back up this inclination. What do you think?

Really, the only thing to really worry about is rust and rot. A steel frame really doesn't "fatigue" (ie. it doesn't soften with age).

Frankly, I'd take an old lugged frame with nice tubes over a cheapo new frame any day.

531 is 531. As long as the frame is in good shape vis-a-vis rust and overall abuse, it's going to be equivalent to a modern 531 frame, at least on any human time scale.