The coastguard said it was contacted by the Police Service of Northern Ireland (PSNI) and ambulance service just before midday.
The Northern Ireland Ambulance received report of the incident after 11am and declared a major incident after 1pm.
It is understood the group of young people are from England.
Local MLA Colin McGrath has been watching the rescue unfold.
The mountain rescue team said: "At the minute there is no one in difficulty because they are all being looked after".
Rescue helicopters were tasked to a major incident after a group of Army Cadets got into trouble in the Mourne Mountains..
"Most of those are being walked down the mountain to safety, but up to 20 people may require stretcher evacuation, as they are suffering from exposure", the Coast Guard said.
The Coastguard posted on Facebook that as many as 70 people may have got into difficulty.More news: Ballistic Missile Test Launch Set For Wednesday At Vandenberg
Newcastle, Bangor and Kilkeel Coastguard Rescue Teams, and the UK Coastguard search and rescue helicopters based at Caernarfon and Prestwick, were on scene.
Three Coastguard helicopters, three Coastguard rescue teams, police and ambulances are involved.
The three helicopters involved in the operation are now grounded due to the poor weather conditions, however they are "prepared to support casualty evacuation as further information comes in".
It has been reported that a group of teenage British army cadets had been camping in the mountains before getting into difficulty.
"Please God all will be ok".
A Mourne Mountain Rescue spokesman said: "Weather has caught them out".
Up to 17 ambulances are at the scene at Head Road in Annalong, Co Down.