There are several methods available to you. Firstly I like to use a wire brush on the wood, which has the effect of removing some of the softer wood in the grain. Resulting in an old un-smooth texture. Some people also like to bash the wood here and there to create little dents, and other like to use tiny drill bits to make fake wood worm holes.
For the grey colour, I recently found that diluting a black aniline wood or fabric dye, to make a grey colour worked best. You'll have to experiment a bit on some scrap to reach the desired colour. I also read on a model railway forum, that giving the wood a bit of an opaque white coat first, and then staining with grey dye gives a good result of old barn wood. I have yet to try that method though.
Good luck with your project.