Stone landscaping, also called hard-scaping, adds enormous character and value to a home, but stones that are manufactured or freshly cut from larger pieces have a uniform, untainted look that says "new", not old. You can add an aged appearance, however.
Paint the fence with vinegar. Do this on a dry day. The acid in it will create tiny pits in the stone, which in turn will make a home for the microbes that will tarnish the stone.
Stain the wall by randomly and lightly dry-brushing antiquing glaze into crevices, if you want to achieve an aged look more quickly. Don't paint the entire wall. Let dry. Reapply more vinegar, if desired.
Grow moss on the stone. Daub yoghurt or buttermilk onto the wall, especially in the cracks. Patting the yoghurt or buttermilk with a bit of soil helps moss to take hold.
Moisten the fence with a mist of water every day until moss is established.
Reapply vinegar periodically to speed the natural ageing process.
Walls that are in sunny, dry locations will take longer to age and to grow moss.