Brush on the top coat of paint, and use a rag to lightly wipe off some of the paint while wet. The base coat will show through on the areas treated with polyurethane.
Other ways to give wood an aged look are sandblasting and pressure washing. These methods may be used on painted or raw wood, and both remove the surface of the wood, raising the grain and giving the look of years of natural aging by the elements. Sandblasting can be done at monument companies, glass shops and auto-body shops. Pressure washing is a do-it-yourself project: rent a power washer at a paint or hardware store.