It happened around 6:32 p.m. in the 1600 block of Opossumtown Pike.
Police say several of the man's personal items were taken in the robbery. They say the suspects were last seen fleeing the area on Oposumtown Pike near Thomas Johnson High School.
Police say the suspects are described as Hispanic male teenagers, both wearing dark clothing with bandannas over their face. Both teens are described as being around 5'6" tall.
The teenager with the gun is described as having curly hair, and the other suspect was armed with a knife.
Frederick Police are asking anyone with information regarding this incident to contact the lead investigator, Det. Kevin Forrest, at (301)-600-6291.
You can also report anonymously by calling (301)-600-TIPS.
Another man claims he was robbed of cash at knife-point at 40 South Carroll Street around 4pm Thursday in Frederick. He describes the suspects as Hispanic males, wearing dark colored masks.