"Slowing down?"

The other day I met a woman who  decided  on her 80th birthday she would give up driving, and a few weeks before a man who on reaching his 70th birthday decided it was time to” slow down”.
 
Yet both were physically and mentally fit.
 
On the other hand I have two cousins, women farmers in Wales in their mid 80’s, busy running family farms.
 
 And this afternoon walking around our local reservoir I met several  older people who were certainly not “slowing down”. Four people were wild water swimming. 

Why do some people think that on reaching a “big birthday” they have to change their behaviour?