difference—theycannotbestopped.Ourfunctionallung
capacitydecreases.Ourbowelsslowdown.Ourglands
stopfunctioning. Evenourbrains shrink:attheageof
thirty,the brainisa three-poundorgan thatbarely fits
insidetheskull;byourseventies,gray-matterlossleaves
almostaninchofspareroom.That’swhyelderlypeople
likemygrandfatheraresomuchmorepronetocerebral
bleeding after a blow to the head—the brain actually
rattlesaroundinside.Theearliestportions toshrinkare
generallythefrontallobes,whichgovern judgmentand
planning, and the hippocampus, where memory is
organized.Asaconsequence,memoryandtheabilityto
gatherandweighmultipleideas—tomultitask—peaksin
midlifeand thengradually declines. Processing speeds
startdecreasingwellbeforeageforty(whichmaybewhy
mathematicians and physicists commonly do their best
work in their youth). By age eighty-five, working
memoryandjudgmentaresufficientlyimpairedthat 40
percent of us have textbook dementia.
WHY WE AGE is the subject of vigorous debate. The
classicalview isthataginghappensbecauseofrandom
wearandtear.Thenewestviewholdsthatagingismore
orderlyand geneticallyprogrammed.Proponentsofthis
view point out that animals of similar species and
exposure towear and tearhavemarkedly differentlife
spans.TheCanadagoosehasalongevityof23.5years;
theemperorgooseonly 6.3years. Perhapsanimalsare
likeplants,withlivesthatare,toalargeextent,internally
governed.Certainspeciesofbamboo,forinstance,forma
densestandthatgrowsandflourishesforahundredyears,
flowers all at once, and then dies.