Not so sure if I ever revealed my age up here, but I'm 46.
There were two major changes I noticed to my body, when I entered into the 40's -- joints & muscles.
Used to have no problems of working on the engine bay by standing and leaning against the bay for hours in my 30's. but not so much anymore, due to my weak lower back that I damaged when I was younger. Hand washing my car has always been very therapeutic to me, but my knees didn't agree with me last time. The driver was on the limp mode the next day, instead of the car.
I'm getting far sightedness, because I started to prefer moving things on my hand away from my face to get a better focus. Things get blurry and out of focus when they start to get closer than 10 inches to my face. I have a friend who's 10 years older than me, and she tells me that I'll be definitely wearing glasses within a year.