Numerous factors play into the "old look" of antique photos, I think. One of those factors, as has been noted, is the nature and quality of the lenses at the time. One way I've found to simulate part of that look is by placing difusion material over the camera lens.

Here's an example of using 1/4" bubble wrap (packing material) with a hole cut in it about 1/3 the diameter of the lens. It's a scan of a 4x5 Polaroid, toned in Photoshop, but the technique should work on 35mm, too. I just stretched the bubble wrap over the lens, and held it in place with a rubber band.