A 16-year-old who went into an estate agency office produced a hunting knife.
Three terrified employees in the Chase Evans office fled into a rear room while the teenager stole their mobile phones.
The youth was one of a pair of offenders described as “menace to the London public”.
The younger teenager, who cannot be named, and his older accomplice both pleaded guilty to a string of offences, including violent robberies where an axe and a rambo knife were used to terrify victims.
The pair approached the estate agents in Elephant & Castle, on a stolen moped, on January 3 this year.
The 16-year-old and Frankie Batchelor, 18, were both arrested the following day.
The 16-year-old has now been sentenced to three years in a young offender institution, while Batchelor has been given five years in a young offender institution.