My son rode his first bike (a PW50) at 4 years old. He's messed with bikes since he could walk. You have to be careful and wear gear, but everything has some risk.
Here's my son in 1983:
Here he is with his son last week!
It appears their having fun & I wouldn't want to take that away....
It works when you get old too. Here's my wife's grandma
with me in 1983:
Steve in KY