As I understand it, OSX does not work that way because it is not resolution independent. If it were, you could scale up and down at will, and as you say, you could have a computer that's readable for those without 18-year-old eyes, and yet with a huge number of dpi.
Again, only in my limited understanding, since it's not resolution independent, the higher resolution screens make text smaller and smaller as the resolution goes up.
I actually prefer the super-low resolution on my old iBook 14" for this reason. Yes, it's low quality and sort of "blurry," but at least I'm not having to read with a magnifying glass.
You can use Command-->+ to make things larger, but that makes websites and stuff go out of whack.
If there is a better way, I hope someone will post it here. I'm already finding my MBP 13" 2010 to be too "small" in text size for my comfort.