Lets take a look at the materials:
1) Earth tone oil paints, black oil paint, white oil paint. Do not overspend here, just keep it simple.
2) Turpentine. If indoors, get the less smelly stuff.
3) Junk paintbrushes.
4) Hair dryer (optional)
Step 1: Load up a napkin or something disposable with blobs of the oil paints.
Step 3: Dip your brush into some oil based thinner or turpentine and load the bristles up slightly. Place the brush directly on the model and streak the brush slowly downward. The longer the line, the more consistent the aging effect.