Why do some trucking schools require you to have been licensed to drive for 3 years?

Hello there, I was just wondering why do some trucking schools require you to have been licensed to drive for 3 years whereas it doesn't state it on some other trucking schools. I was just wondering if this was the schools own policy due to insurance etc or are you required by law to have been driving for 3 years before you can go for your cdl.

 

18 years old usually means you have graduated from high school. Add 3 years to that and you become 21. Can't drive out of state till 21 years of age. Just gives the driving school graduate more options. I had a 19 year old in my class. He was so immature that I was very afraid of him having a license to drive an 80000 pound vehicle on the road.

 

many thanks for clearing that up