Maybe he can just “walk it off” if he runs over his foot. Just because you did something 50 years ago and you’re here today, isn’t proof that it’s right. Wanna know what’s different today? These kids, they have the attention span of a gnat. He’s going to get bored of mowing in straight lines after about 10 minutes and do something stupid like put it in the fence or in a ditch. But hey, if you want to throw your 5 year old on machine, not a toy, and put them to work, go for it.
Even at the amusement parks they have signs “must be this tall to ride on this ride”. The kid can’t even reach the controls. But hey, that’s how you did it, and you didn’t die, (but others did), so go for it. Why not let the kid drive himself to school while he’s at it.