Honestly, I don’t like it either. But it does make sense. Some health risk factors change drastically between 30 and 40 (e.g. heart disease, colon cancer), suggesting a kind of dividing line between “my body just does what it’s supposed to” and “what, I have to do extra stuff to survive now?!” A lot of people also change rapidly in physical appearance during this time too, exhibiting profound male pattern baldness, love handles, belly fat, winkles, etc.