The force helicopter tracked the alleged offender after receiving the call while attending a suspected burglary in Exeter.
The young victim is understood to have been injured with a piece of glass by a 13-year-old boy in Chestnut Avenue, Wonford.
A spokesman for Devon and Cornwall Police's air operations unit said: "At about 4:10pm on Sunday, October 7, the force helicopter was deployed to a report of an assault in Exeter involving a slashing with a piece of glass.
"The aircraft which was airborne over Exeter on another task was quickly on the scene and located three persons answering the description of the offender.
"The crew followed the suspect and directed ground officers to the suspect's location."
A police spokeswoman said the offending youth was dealt with by means of restorative justice.