At around 8am on Tuesday, 13 October, the victim was delivering newspapers in Northlands Gardens when he was approached by two men, one on foot and the other on a bicycle.
The man on foot threatened the victim with a knife whilst making demands for the mountain bike before taking it and riding off.
The man on foot is described as being white, 18-years-old, 6ft tall, of average build with light brown cropped hair in a perm. He was wearing grey tracksuit bottoms with a Nike logo and a grey hoodie with the hood up.
The man on the bicycle is described as being white, 18-years-old, 6ft tall, of skinny build with short brown hair that is shorter on the back and sides. He was wearing plain grey tracksuit bottoms, a grey tracksuit top with the hood up over his head, a glossy black puffer vest over the top and a black face mask.
Officers have released this photo of the stolen mountain bike which is described as a red Scott Aspect mountain bike with SR SUNTOUR written on the front suspension in black. It had a white LED on the front in black casing.
Have you seen this bike? Did you witness the incident? Perhaps you were driving in the area at the time and may have some dash-cam footage?
Anyone with any information about the incident or the bike in the picture should call 101 quoting 44200397118 or call Crimestoppers anonymously on 0800 555 111.