The man was flown to Southampton General Hospital to be treated following the crash, which happened on La Pulente Hill at around 03:40 on Sunday morning.
His condition was initially labelled “serious”, but the Health Department have since confirmed that he is now “stable”.
He remains in the hospital’s Intensive Care Unit.
The 18-year-old driver, and two 19-year-olds who were also in the car, came away from the crash with minor injuries. No other vehicles were involved in the incident.
Jersey Police would like to speak to anyone with information on the car leading up to the crash and its occupants.
Anyone with CCTV or dash cam footage is also being urged to contact police on 01534 612612 or anonymously via Crimestoppers on 0800 555 111.