Shakuni was Gandhari's brother and hence was alive before Pandavas-Kauravas were born. Now refer following narrative:
"Sakuni said,--'One that is intoxicated falleth into a pit (hell) and stayeth there deprived of the power of motion. Thou art, O king, senior to us in age, and possessed of the highest accomplishments. O bull of the Bharata race, I (beg my pardon and) bow to thee. Thou knowest, O Yudhishthira, that gamesters, while excited with play, utter such ravings that they never indulge in the like of them in their waking moments nor even in dream.' [Sabha Parva SECTION LXIV]
Why did Shakuni call Yudhishtira older than him in age?