In Time shows a world where time is currency. The film explains that on a person's 25th birthday, their clock "starts ticking", with one year on it. From that point forward, they can buy and sell things by exchanging time with other people. And when a person's clock runs out, they die instantly.
If this 25th birthday gift of a year was the only source of time, the average life expectancy of everyone (who didn't die prematurely) would be exactly 26 years old (or less, if we account for accidental deaths). The movie makes it apparent* that even in the "ghetto" (where people often live with less than a day on their person), many people are older than 26 years old (Will Salas's mother is an obvious example).
Do we know anything about where this extra time comes from? Is more time injected into the system somewhere?
*I suppose it's possible that the average life expectancy is 26, and that the vast majority (meaning literally millions or billions) of people die soon after their 25th birthday (and the movie just never spoke to this point) allowing the wealthy to accumulate such vast wealth.