“Boy, this year sure went by fast!” is so tempting to say because each year is perceived in terms of chunks at a higher, grander, larger level than any year preceding it, and therefore each passing year contains fewer top-level chunks than any year preceding it, and so, psychologically, each year seems sparser than any of its predecessors. One might, somewhat facetiously, symbolize the ever-rapider passage of time by citing the famous harmonic series:
1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 + 1/8 +...
by which I mean to suggest that one’s nth year feels subjectively n times as short as one’s first year, or n/5 times as short as one’s fifth year, and so on. Thus when one is an adult, the years seem to go by about at roughly a constant rate, because — for instance — (1/35)/(1/36) is very nearly 1. Nonetheless, according to this theory, year 70 would still shoot by twice as fast as year 35 did, and seven times as fast as year 10 did.
But the exact numerical values shown above are not what matter; I just put them in for entertainment value (who else but Hof and D.B. would throw in harmonic series for light relief?). The more central and more serious idea is simply that relentless mental chunking makes life seem to pass ever faster as one ages, and there is nothing one can do about it."