Here is the website for the "Tennessee Government Department of Safety & Homeland Security" http://www.tn.gov/safety/driverlicense/classmate.shtml
. Here is what they say:
Motorcycle Operator (Class M) License
What is a Class M Motorcycle Operator License?
A Class M motorcycle license allows the holder 16 years of age or older the privilege of operating a motorcycle or motor-driven cycle over 50cc. The operator of a motorcycle or motor-driven cycle will need to apply for a Class M license (or XM license). These two and three-wheeled vehicles are categorized by cylinder capacity and other design features. Class M licenses (XM license) may be issued as a single license, or along with another primary class.
Applicants wishing to apply for a Class M license/XM license should obtain and read the Motorcycle Operator Manual which is available at all driver license service centers located throughout the state.
Any operator of a motorcycle or motor-driven cycle 15 years of age requires a special permit allowing the fifteen year old to operate a motorcycle with certain restrictions after completing the requirement established for a motorcycle license.
Anyone with a valid Tennessee driver license may operate a motorized bicycle that is 50cc or less and no endorsement is required on the driver license.