This varies greatly from one person to another. I had worked in long-term care facilities for over 30 years, and during that time I saw people who were worn out by 50 (even younger) and then others who were still going strong at 80+. Genetics obviously play a big part in how we age — if your genes are strongly against you, your best efforts to remain healthy and active may not be enough (I think of the women in families with a strong history of certain cancers, who then find that they inherited a gene that predisposes them to developing breast cancer, and they choose to have prophylactic mastectomy to reduce their risk. I also think of Jim Fixx, whose father died in his late 30’s from arteriosclerosic heart disease. Fixx took up distance running (in addition to diet) to try to prolong his own life and in the process he became a noted authority and authored books on running. Still, at age 52, Fixx died of a heart attack while running. He likely lengthened his life by the changes he made, but he could no out-run his genetics). Then, there are the folks who smoke, drink alcohol to excess and eat saturated fat, and they are reasonably healthy into advanced age.
Still, your lifestyle and habits also have impact upon health. If you keep a moderate weight for your height, eat a varied diet that includes the things we know are good for you, and get regular, moderate exercise, that is a good start to maintaining heath. If you do not use tobacco, consume alcohol lightly or not at all, do not abuse prescription medication or illicit drugs, use sunscreen when outdoors, see a doctor regularly to detect and treat health conditions early and before they become bigger problems, provide daily oral care for yourself and regularly see a dentist, those are all things that positively affect health. If you take reasonable precautions while driving and going about your day, and avoid activities that are known to be risky and conducive to injury, then you have done a lot to promote your own health.
A positive attitude about life, people and daily challenges is also very important for health. I don’t have a ready link to support this, but I think it is safe to say that people who hold a positive attitude toward life tend to live longer. Stress is part of life, and managing stress is a good skill, while too much stress is harmful both physically and mentally. Many people (myself included) tend to ignore and take for granted the many positives that each of us have in our lives. Our health and abilities (could be worse, but isn’t), our family and friends (some gone, but others still here), our ability to work and earn a living (some people can’t work, and some have less than we do) are things to be thankful for. Yes, we have sad things that happen in our lives and cause us disappointment, and we may need to grieve those things, but then we need to remember the good that we still have and get about living again. Life tends to run in cycles of good and bad — we should try not to get hung up on negative thoughts and risk missing the positives that come. People are social beings, and part of longevity is having fun and looking forward to things, and having other people to populate our lives and have fun with and provide mutual support with in trying times. Seek to be active and socialize no matter your age or abilities.
Another aspect of reality is that life brings changes. Getting older means increased risk for disease, disability and death. Hopefully, if you mostly pursue health choices and your genetics are not too wicked, your health problems that tend to increase in your 50’s will be few and minor. It is very important to take care of yourself and deal with health issues early. The human body is more forgiving of abuse and neglect when you were younger, but you can’t get away with that as you reach your 50’s (or even 40’s in some things). If you haven’t cared much about your health and took it for granted before, now is the time to change bad habits to good ones. Also, things will change socially whether you want them to or not. Things that I noticed as I neared 50 were that younger people seemed to care less for my company or to value my opinions. I had become “old” or “older”, and my ways of thinking about and doing things were seen as less relevant. As most of my life was behind me, I did talk mostly about the past and how things were and were done — the young often don’t value that. So, I had to realize and learn to accept that I was becoming less relevant and valued, at least in some circles. I could relegate myself to only hanging out with the oldsters, or I had to listen more to the young and treat them more as my equals if I wanted some acceptance from them. On some things, I chose the latter — it helped that I have a lot of child in me that enjoys mischievousness (But, don’t think such acceptance means that you can date much younger than yourself — you are still old and a bit deteriorated, and what young person wants to touch that?) It never hurts to try to establish some connection with younger people — take a college course, learn a new skill or hobby — as they can be part of that people connection that we all need and want.
To get back to the question: physical decline usually starts before age 60, but you have a lot of say in how your older years will go. The decisions we make at any age have great impact upon our life at any age, and old age is no exception.