I got my DL as soon as I could and bought a car as soon as I could - as well as being able to afford the insurance. My dad would not let me on his policy and forced me to get 100/300 on my own policy so long as I lived at home. I moved out at 19, but still kept the 100/300.
My kids were the same, but I bought their cars. I made them get their own 300/300 because that's what my umbrella required (and still does).