I was making about 20 bucks a week at age 11 cutting lawns - of course that was summer 1976.
I think a responsible kid of about that age can certainly use a regular push mower if they've been properly taught by an adult.
As a rule, age 9 is still too young (though I'm sure someone will say they cut lawns at age 9 unsupervised, but as a general rule that's too young)
I agree (from the article), 12 is entirely too young to be using a riding mower. 16 should be the min. age for a riding mower - assuming they've been properly taught.