I'm sure psychologists have studied this issue, but common sense would suggest that older people tend to live in the past, whereas younger people tend to live either in the moment, or for the future.
That's a generalization, of course, and I'm sure individuals have "cycles" of sorts. For instance, at the end of the calendar year, it's probably more likely to think about (or relive) the past year; on the other hand, don't a lot of us think again to the new year and the promises it holds? That's why people make New Year's resolutions after all.
Life must be lived in a continual series of "nows," and to a certain extent, living too much in the past or too much for the future is like not living at all. In other words, it's the -ing -- the now-ness -- that we should pay most attention to if we truly value living.
What's the most effective way to live in the now?
"I wasted time, and now doth time waste me." -- William Shakespeare