In many countries today, between 70 and 90. In the 22nd and 23rd centuries it was around 100. In the 24th century it was about 120. Some humans have lived longer, such as to 130, 140, or moar.
In "Encounter at Farpoint", Admiral Leonard McCoy is said to be 137 years old. Ironically, DeFrost Kelley was the first main cast member to die.
I would think that human life spans would not be able to exceed around 125-130 years but it is the future...
Also Kirk did not seem to age while he was in the extradimentional nexus, but this of course is outside of regular space time
A few people, the so-called supercentenarians, have already lived into their 110s and at least one to 121.