You must be 21 years old or older to drive a vehicle with alcohol in it.
To transport alcohol in a car or other vehicle legally you must be 21 years of age or older.
Since you have to be 21 years old or older to purchase and possess alcohol it's the same with the age you have to be to transport alcohol.
However if the alcohol was in the trunk of the car or in a locked tool box then you could get away with transporting the alcohol when under 21 years of age.
That is as long as you're not drinking it and have no alcohol on your breath.
The police would need your consent to search the toolbox or trunk without a warrant.
So if you need to transport alcohol when under 21 years of age make sure it's sealed and in a trunk or toolbox that is not seen.