That's a great question. I do know that lichen is super slow growing and will only grow on something that has been well-established for a number of years. Rates of growth can vary from 0.5mm per year to 500mm per year depending on a number of variables (climate, lichen species,symbiosis). I am not sure what the growth rate would be in the Intermountain West but would venture a guess around 0.5mm-1mm per year based on our climate which would make your shed pretty old... Just my 0.2