I just got to bite on this one...
Perhaps a better tittle would be "how old is old enough"?
Bunch of us have grown up around machinery, we know it, we understand it, we respect it, we have seen it take loved ones from us.
I have a very deer friend (52 years old) whom I would not trust with a weed eater...he will either hurt himself or someone else EVERY time.
Some folks just are not able to opperate machinery of any kind reguardless of their age..
That's OK, I am not able to do lots of things... fill out a drug prescription, do my taxes, build a large building, bla bla bla.
Some folks seam to have a gift about machinery.. there is nothing out there that they are not able to master in just a few hours.. I am not one of those folks.
My baby boy (24 years old) is able to climb onto just about any machine out there, preform a job without abuse and get it done right the first time..he is a whiz with hand tools, can think ahead as far as construction scheduleing and only has a highschool degree. My oldest is 27, collegue educated, and is a dangerous person to be around when he mows the grass.. but can he ever do financing / banking / fiscal forcasting... paper work is his domain.
everyone is differant., know your limits, and the limits of the machine.