In general, to apply for a CDL, an applicant must be at least:

In general, you must be at least 21 years old to obtain a CDL in Tennessee. You may obtain a Class B CDL at age 18 if you will only drive in-state within 100 miles of your job and require no special endorsements. At age 19, you may obtain a Class A or B CDL, but you may not have any endorsements.
DMV Writen Test