At what age should a child be allowed to sweep chimneys? Your personal opinion required?

As soon as they can climb - you can't leave it too late or they become too large to squeeze their way up. My ancestors worked in the pits from the age of 9, as coal hauliers (8-hour shift and had to walk 4 miles to the pit and back at night), so I guess by six or seven they would have been considered old enough and sensible enough.