Looks like 6 is too young.  Dad and boss' dad thought we both started about 8 or 9.  He's been running the skid steer for a couple years already moving stuff around, pulling fence posts and other misc stuff with me right there and does really well.  Even leveled off some gravel and dirt with me in it last week.  Lawn mower is a little different, no roll cage or anything.  I'm a firm believer in starting him young on this stuff so he knows not only how to run machinery, but responsibility and work ethic as well.  We'll hold off for now.  Thanks for the replys.  Ben