The reality is that almost none of this stuff comes on "suddenly." Every day when we use our bodies and minds, we are making hormones and moving muscles and bones and ligaments and tendons and firing neurons and all of that. And if something is a little bit off, it maybe scrapes a teeny bit, or leaves some hormonal need just slightly undersatisfied. And that accumulates over the course of your life, until at some point it reaches critical mass and you notice it. Things that I have had "suddenly" go wrong between 40-50:
-- Eyesight: all of a sudden I need reading glasses. Except when I finally gave in, I realized my arms had been getting shorter and shorter for a while now; it was just so slow that I didn't notice it.
-- Joints: I started having knee problems. Turns out I don't walk "properly," and so my kneecap doesn't follow the right path and has worn down the cartilage and caused a lot of inflammation and pain. And my rotator cuff: turns out that when I hurt it in college, I really should have gone to a doctor, because now my lats and pecs have learned to compensate, and that has caused all sorts of other issues. Oh, and best of all, my back: I apparently have a curve in my back -- not scoliosis, just years of bad posture. So now I can't run, because my back seizes up.
-- Hormone stuff: menopause hits in ways that you might not even expect. The change in those hormones in your body can affect other hormone levels and body processes, too. In addition to the obvious/much-talked-about symptoms, over the past @2 years my blood pressure has shot up (from 90/60 to 110/75) and my cholesterol has suddenly spiked over 200. Same diet (if not better), same exercise (if not more).
-- Teeth: all of a sudden I had gingivitis and cracked a tooth and needed a crown. Except the gingivitis was from decades of absolutely hating flossing, and the crown was because I have been grinding my teeth at night for years.
IOW, you can probably guess for yourself where your problems may be. Your 40s and 50s are when all your bad habits start to come home to roost. So do what you can to take care of yourself:
-- Maintain flexibility and balance. Yoga, stretching, whatever. It sounds stupid, but I always put one shoe on while standing on the other leg, just to test my balance.
-- Maintain strength. I was stunned when I couldn't do a cartwheel a few years ago -- I mean, I ran and did cartwheels and handstands and everything as a kid, but then I tried for the first time in years, and my arms couldn't hold me up. When did that happen?
-- Remember that your heart is a muscle too. Get your heartrate up on a regular basis.
-- Eat better foods. As the coach at my gym says, be a grownup, do it because you have to.
-- Go to the doctor and dentist regularly. Many of the physical/hormonal changes come with no definable symptoms, so you will find out you have a problem only if they do a blood test. And for the love of Pete, get mammograms and colonoscopies and all of those icky things we all love to avoid.
-- Recognize that you cannot recover as quickly any more. This is far and away the hardest for me; when I get injured or sick, I hate waiting and waiting for things to get better and usually push things too quickly. But when something goes wrong, your body needs rest to recover. So suck it up and rest. [I should note that my top goal at the gym this year is "no dumb-ass injuries"].
-- Continue to push yourself to try new things. Just as our bodies calcify when we don't use them, our brains do the same thing when we do the same thing over and over and over. Ruts are death. Take a class, try something just because you know you'll be bad at it, fix a leaky faucet yourself, face down a fear, learn everything you can about an issue that interests you, etc. -- the "what" doesn't matter.
All of these things may or may not lengthen your life. But they will certainly improve the quality of whatever time you have.
Finally, and less specifically: accept your limitations and focus on what you can do instead of what you can't. This is the time of life where it is easy to get down on yourself, because when all these things start going wrong, you realize that you're not as [insert adjective here] as you used to be, and that things are only going to get worse, and from there it's just a hop-skip-and-a-jump to life-sucks-and-then-you-die. Listen to me when I say: that is all bullshit. You can do more than you think you can. You know that cartwheel I failed at? Well, it sent me to CrossFit, and now I can lift more than I could in college. I've been a lawyer for 25+ years and learned an entirely new area of law for my job. My boss gave me a nameplate for my office that says "unfuckwithable." Yeah, I've lost some things, but I've gained a lot more. I am way more of a badass now than I ever was in my youth.
That is why you need to push yourself to try new things: because it forces you to realize that you are capable and strong. And then maybe you'll spend less time on message boards fretting about all of the parade of horribles that are going to come your way as you get older. ;-)