In fact, it's about brain metabolism. It slows down with age, so people get depressive progressively (Meds use the term that is best translated as "age depression").
Jimbo is on weed, so this might play a key role, since it's anti-depressive as well. I can also report that i became far less grumpy than my long time friends of similar age, especially compared to those who don't take weed.
I'm not even near Jimbo's age, but in my mid 40's, but i know quite some potheads in the age of 50-65 who support my observations.
Ah yeah, and a young wife. Mine is about a decade younger than me