For the Finder and Desktop choose View Options from the View menu. In Safari preferences click on the Appearance icon. Click on the Advanced icon where you can set the minimum font size. Most applications should have a means for changing display fonts. Maybe in their preferences or a Format menu. You just need to explore to find it, if it's available.
The zoom feature of OS X is OK, but it has limitations such as loss of sharpness and the need to scroll around the screen. Not quite the same as using a lower display resolution which may only reduce sharpness depending on how much lower you make the display. LCD displays show some lower resolutions better than others, but usually not as well as their design resolution.
Might also take a look at these:
SizeWell
RightZoom
CNet Downloads or
MacUpdate. They may be helpful, but I don't know from any experience using them.