I used balsa wood or popsicle stick to construct building floor for Miniart 1/34th ruin building.but how do you make the wood look worn and aged?

I thin lacquer or enamel 1:1. You can then paint the wood with that. Vary the paint by a shade and apply, with lateral brushing. Don't cover the entire timber, with this second coat. Again, vary the shade slightly and apply in the same manner. And, again not over the entire timber. For old wood, use grays. For fresher areas, use browns. I don't know of any way to do it, with an airbrush. You may also rub some pigment(of the appropriate color) into the timber.

search with "weathering wood modelling" on YouTube - there are a lot of excellent video-tutorials

Expose it long enought to rain, wind and sun and it will age themself.