If you're asking about health...
Losing perfect vision. Needing reading glasses. People start to getting presbyopia as early as age 35, and it hits most people in their 40s.
Metabolism changes. My habits haven't changed, but it's much more work to not gain weight, for me.
Changes to my perception of the passage of time. Things that happened years ago seem more recent. Time seems to pass faster. I've read once (I don't know if this is true) a younger brain records or recalls more of your day-to-day activities and so your perception of time is slower because you remember more of it.
Yeah, there's a reason why, when you talk to old people, some of them go on about their health problems. You don't really think about your health when everything is perfect for you.
If you're asking about culture and society...
The rise of terrorism.
How fragile our democracy (America) can be. That's a realization coming to me since November.
Casual attitudes about, and pervasive use of, hard drugs among younger people. I'm not talking about beer or marijuana.
For most other things, nothing really surprises me. Sometimes things disappoint me, but that's a far cry from being shocked or offended.