The incident happened in the Deans Walk area of Lurgan, just before 9 pm on Friday.
Another teenager has been arrested and is helping police with their enquiries following the incident.
A spokesman for the NI Ambulance Service said the teenager received treatment at the scene before being taken to Craigavon Area Hospital.
The street was closed by police while the scene was examined.
Officers have appealed for witnesses to come forward.
A PSNI spokeswoman said the teenager’s condition was “critical”.