The average cost of car insurance for a 20 year old will depend on whether the person is a male or female and what make and model and year of vehicle the 20 year old is insuring.
If you're a new driver your insurance rates will be higher at least until you turn 25 and keep a good driving record and then your insurance rates may drop down overtime.
At 20 years old you can expect to pay around $80.00 to $200.00 per month for liability insurance and if you want or need full coverage auto insurance then you could pay as high as $250.00 to $350.00 per month depending on the value and make and model as well as year of vehicle you're insuring.
The only true way to know what the auto insurance will cost you personally or the 20 year old is to look online at auto insurance websites and get some free quotes and then you can even purchase auto insurance through the insurance companies website.
Check different insurance companies websites and get different quotes because some insurance companies may be more expensive than other insurance companies when insuring you and also your credit score can also factor into the cost of your insurance costs.
The higher insurance and Credit Score you have the lower your cost of auto insurance will usually be and the lower your insurance and credit score is the more your insurance will cost you.