Hey,
For a production I have to start with a clean new location and I have to age this
location. The shot is going to be of a man sitting on a chair in the middle of an old hall.
There is only going to be 1 top light so I don't have to mind the environment around him because
it will be dark.
But I have never done something like this before so any advice would be great.
I was thinking of mixing some white fine sand with some white flour and spread it around.
Then adding a bit of smoke.
But are there better ways to create dust or things like that ?

I don't understand your question. The surrounding area will be dark so the environment isn't a consideration? So do you mean aging just the floor? Or just the chair and person sitting there?
White flour is too white to look like dust on screen. You need Fuller's Earth, or maybe whole wheat flour. Even whole wheat flour might be too light on screen, please test. Here is a tutorial on how to make your own cobweb gun using rubber cement. Skip to 3:40 to see the results! After you make the cobwebs, lightly dust it with powder to bring it out in the light.
Make cobwebs for film or theatre production - YouTube

Would a cookie work for what you want?

I love Film Riot's technique of a branch with leaves in front of a light. Skip to 6:30 for the entire section, or 8:21 for the branch part and see what it looks like.

Cinematography Tips: Breaking Up & Diffusing Light! - YouTube