Q: I was recommended by a nursery to ask you about a solution that can be applied to outdoor cement walls to make them look aged. Can you tell me what to use?
A: Anything that sticks to stone (or concrete) and adds slowly-dissolving nitrogen to the surface will encourage molds and mildews to grow there. These give the surface an aged look.
There are lots of recipes: manure tea, buttermilk, gelatin, etc. I can’t say which is best but they all will work. Just mop the stuff on the surface and keep it moist for a few weeks.