import arcpy, os
from arcpy import env
arcpy.env.overwriteOutput = True
env.workspace = r"D:Test"
outraster = env.workspace


walk = arcpy.da.Walk(env.workspace, topdown=True, datatype="RasterDataset")
for dirpath, dirnames, filenames in walk:
    print dirpath
    rasterList = []
    for file in filenames:
        raster = os.path.join(dirpath, file)
        rasterList.append(raster)
    try:
        rasMean = arcpy.sa.CellStatistics(rasterList,"Mean")
        rasMean.save(os.path.join(outraster,"mean.tiff"))
        print rasMean

    except:
        pass