the same reason people are bothered by so many other things.

they freak out at their lack of control and they wish they could control things. especially how they are perceived by others... hence being reminded you are OLD is offensive, because OLD is bad, and YOUNG is good.

and a lot of folks are so desperate for a sense of control they are easily conned by hucksters who sell them on the idea they can prevent cancer by popping expensive supplements or going on $15,000 yoga retreats.

a lot of our economy is built on preying on human insecurity and it's anxious need for perceived control of our bodies, other people, and our environments.