Reclaimed Stone
Your retaining wall will look as old and weathered as the stones used in it. For an aged look, used reclaimed stone instead of newly cut rock. According to Bob Vila.com, you can make use of old granite from weathered walkways or sidewalks to build your retaining walls. The granite will already have the worn look you want and, if it is cracked and broken into irregular pieces, will look even more rustic.
Coatings
According to This Old House online, new stone is often sprayed clean with muriatic acid. This removes organic materials from the stone, making it hard for organisms like mold and moss to move in and give it a weathered look. You can correct this by blending topsoil and compost together and mixing them with water to make mud. Spread the mud on your stone wall and allow it to dry, then wipe or brush most of it off, leaving the stone with a thin layer of dirt. This dirt will encourage the growth of organisms that eventually give stone a weathered look.
Distressing the Stone
New stone retaining walls have smooth, even edges and perfect seams which give away their lack of age. Distressing them gives them a head start on the weathering process. Bob Vila.com recommends torching the sides of granite or stone blocks and wearing them away with a chisel to give them an aged, weather-beaten look from day one.