The average Human life span had gradually increased during their history largely in part to advancements in nutrition, medicine and technology improving their health and overall quality of life. At one point, it was only thirty-five years. (ENT: “Similitude”) That average roughly doubled by the 20th century. During the 22nd century, it extended to about one hundred years. (ENT: “Observer Effect”). It remained largely the same through most of the 23rd century, though it wasn’t unheard of for people to reach their 120s. (Star Trek) By the mid-24th century, Humans often lived to be 120 years old or more, with the oldest living Human (Leonard McCoy) living to be at least 137 years old. (TNG: “Encounter at Farpoint”)
It’s about 120 on average. From cannon.
Where’d you get that number?
According to Memory Apha: