I'm in my 50s and these are mine..
1) Almost everyone gets married, so when you’re in your 20s/30s thinking it will never happen, it probably will. Haven’t seen much infertility but maybe that’s still to come given age.
I have three friends who are in their 50s who have never been marrid or were divorced a long time (20+ years) ago. Not much infertility in my group of friends though some have chosen to remain childless.
2) Seen two affairs, both ended marriages with kids
Couples I always thought were solid couples are getting divorced and i'm wondering if now that the children have left home there's nothing to keep them together?
3) Mental health is a massive thing in lots of people’s lives - I think there is a lot of responsibility when you have young kids, need to earn etc and that’s often how the stress comes out, hoping this will get better as kids get older etc. Also know a few people our age with serious physical health problems.
It's a massive thing and i know a number of teens who are having counselling including my own DD. I always thought it would be the sleepless baby days that would break me but now i'm convinced it will be the sheer 24hr exhaustion of my daughter's depression.
Sadly DH lost a school friend last year and another dad friend has just died suddenly. Both fit men in their mid fifties 
4) Interestingly with all the Caroline Flack stuff in the papers (not saying she did anything) we have known two cases of wife on husband DV, no husband on wife cases
I've only known male on female - a friend of my dad's though the wife finally managed to get out of the marriage after 15 years of hell. It was only at that point we found out.
5) Re my parents and their circle, health got quite wobbly for a few people around 60 so good to have financial options
My FIL died in his mid seventies, closely followed by the deaths of 5 older friends (all cancer). My parents have a number of health issues but are struggling on in their 80s. They have the money for a good retirement but are unable to enjoy it and can't travel. It's heartbreaking to know that my dad will never see his birth country again.