May not be perfect but it certainly would be worse if every 13 yr old kid was able to drive, especially without any driver ed. As far as insurance, making it a legal requirement and making proof of it mandatory for getting plates etc means that a higher percentage of drivers are actually going to be insured than would be otherwise.
No one ever said a law was going to lead to perfection. Libertarians, however, just assume that if a law cant immediately solve a problem than you might as well decriminalize the behavior: legalize drugs, prostitution, and driving under age and without insurance.
Moreover, most libertarians would still be ok with local enforcement of these issues, what libertarians primarily hate is the federal government getting involved.
Forget about 13 year olds. A driver's licence test is done at (typically) 16 years old and then the state just demands a fee every so often after that. It's about money (and control), but not about safety. If it was safety, you'd be tested more frequently.
What part of ingesting drugs, having sex (with a consenting adult), and operating a motor vehicle is so inherently evil and dangerous that it must be illegal or you must be licensed to do it?
PS - I don't do drugs, do not frequent prostitutes, and rarely allowed my 13 year old to drive.