Born and raised near Mardin, Turkey, Kosen's growth as a child was normal until the age of ten when he was diagnosed with pituitary gigantism after doctors discovered a tumor behind his eyes. The tumor had put excess pressure on his pituitary gland and caused his extreme growth. He underwent a series of surgeries and his growth continued until the tumour was removed in 2008. His condition has caused other medical problems. Kosen suffers stress on his knees that necessitates the use of walking canes for most activities.
Kosen's height was recorded at 2.47 m (8 ft 1.2 in) in his home country by Guinness World Records, at August 25, 2009, overtaking former world record holder Bao Xishun who stands 2.36 m (7 ft 8.9 in) tall. Kosen is also the current Guinness World Record holder of 'largest hands', and 'largest feet', measuring 27.5 cm and 36.5 cm respectively.
