Someone mentioned seeing their family members get older/die. I think that's the primary one for me. 3/4 grandparents are still alive in their 80s, 90s and for the first time in a while, I can look at my parents and see them actually look like "senior citizens" as they approach 70. They still have their health, no serious issues, go out, can move around and all, but still -- I see people that most would consider old now when I look at their faces. At 36, I don't have children yet, so I wonder if all of the people mentioned will be able to have a relationship with them when I do so.
Death, man.. my best friend of 30 years died last year. That was the first time in my life where someone I truly and deeply cared about died and it wrecked me physically and emotionally for a year straight. The thought that as i approach 40, more and more people will start dropping like flies is terrifying. I know that my 40's are gonna be the decade of me gaining so much financially and relationship wise with my fiance, but I also know that that'll be a dark decade when it comes to others I love.
I don't worry about myself too much. I've never ever been flabby and sick and I'm way too resourceful and creative to be poor or have a soul draining job.