Important brain functions can keep getting stronger well into old age and Speaking words of wisdom: "This is interesting. Is it because our thinking gets more complex that we see more possibilities? Or did they offer some other theory for the increase in positive and future-focused language?"
Here is my answer:
According to Barbara Strauch´s book The Secret Life of The Grown-up Brain, older people are just as easily capable of detecting negative information as young people but they focus more on positive information. This focus is partly conscious, partly unconscious. Researcher Mara Mather found that the brightest brains do indeed have the most bias toward the positive (which seems to support what you propose).
Two explanations are given by researcher Laura Carstensen as to why this is so: 1) as we grow older we already have a lot of cautionary knowledge about life so there is less need to watch out for negative info than young people, 2) we have a conscious and unconscious awareness of their time left in life. "When time is perceived as expansive, as it is in healthy young adults, goal striving and related motivation center around acquiring information. Novelty is valued and investments are made in expanding horizons. In contrast, when time is perceived as limited, emotional experience assumes primacy."
In this shift of focus from the negative to the positive it appears that the frontal cortex plays an important regulatory role. Researcher Joseph Mikels found that older people who emphasize the positive the most use their frontal cortex the most. Mara Mather did an experiment in which she distracted older people (through which she put the frontal cortex to work which is the part of the brain which is used to deal with distraction) and found that the positivity effects disappeared. The older people were no longer capable of focusing on the positive anymore.