Emergency service were called to High Street on Sunday (May 17) at about 7.30pm following reports of the incident.
Medics treated a man at the scene for a head injury and he was taken to a London hospital, where he remains in a stable condition.
Officers believe that the victim was punched by an unknown person before he fell to the floor.
The suspect is described by Kent Police as a white boy, aged between 13 and 16 and clean shaven.
He is described as wearing blue ripped jeans, black trainers, a black baseball cap and a white logo and a black hooded top.
Anyone with information, including CCTV footage, should contact Kent Police on 01474366149 quoting 46/7824/26.
