What is the best or worst thing about getting older?
I can only speak for myself, although I am fairly sure that my response is true in general for most people.
The worst aspect of aging is the physical and mental decline. Even those people who are genetically lucky and who maintain a high degree of healthy eating and exercise cannot at 60 or 70 perform at the level they did in their 20s and 30s. When a boxer or golfer or baseball player gets beyond the the usual age for retiring from the sport, it is considered newsworthy enough to be mentioned most times when the player is discussed. And even those who have no issues with dementia often find themselves searching for words that once came easily or more frequently trying to remember why they walked into a room. I am not talking about people for whom either mental or physical decline interferes with their pursuit of an independent life, I just am saying that even in the best cases things are a little more difficult in day-to-day living. Joints are stiffer, bending over is just a tad more difficult - more tiring, or a bit painful, or just requiring more effort. It may stop you from doing nothing you enjoy, but it just makes both mental and physical endeavors more difficult and more tiring.
I am sure that many would say that the losses one has experienced of loved ones from death or life's vagaries is the worst aspect. These are difficult and leave unfilled empty spots in one's life. It is rare that people in their 70s and older feel that deep and fulfilling friendship and closeness to people whom they met after 40. People with whom I slip into conversations with greatest ease are those I met in grade school or high school, college or my first few jobs, or people that I met during those same periods of my life even if they were not classmates or colleagues. Meeting a friend from high school with whom I haven't spoken in years leads to far easier and more comforting interactions than those with people I met after my younger years were finished. More effort must be put into keeping later friendships going. Even close relationships with fellow workers at jobs I took after 40 (and 40 is an estimate, not a hard number) lapse completely if the two of us do not make an effort to stay in contact, whereas I seem to be able to pick up where I left off with the friends and cousins of my youth.
I miss the enthusiasm and the passions of my youth. I have things I like very much and some few things I still believe in fairly strongly, but the excitement and engagement is less in almost everything.
As to the best thing, I think that came for me with retirement: an acceptance of who I am. I love the control over my own life that I have now. No vacation need be cut short, or scheduled inconveniently, no event need be left before I wish to leave because of work tomorrow. I never liked working, but I realize now that I often liked the actual work well enough, I usually had great co-workers and I had more than my share of really great bosses. I hated the regimented life - up at a certain hour, at work no matter what was going on outside for specific period of time. Really, the only thing I truly hated was the ‘have to do this' aspect of my working years.
In my case retirement has coincided with a level of self-acceptance that I think most people find in old age, whether they are retired or not. No, I am not going to read the Complete Works of Shakespeare because I DON'T WANT TO. I would like to be more intellectual, more concerned with getting the details right, more willing to maintain my own car and house. I would like to be neater. I would like to be a lot of good things, but I am not and I am neither going to pretend or try to be what I am not anymore. And I just don't feel bad about these things despite feeling I wold be a better person if they were true. I am not going to events that I once attended from a feeling of duty or from a feeling that I wanted to be the kind of person who took part in such things. I do things now for the person I am, not the person I wish I was. Self acceptance - or the acceptance of the way things are in cases where I am not going to work for change - gives rise to contentment. Contentment is one form of happiness that seems to be denied to most young people. It is a very placid feeling. Such feelings were far more fleeting when I was young. I liked a lot of things more, but I didn't have that lovely warm bath feeling that I describe as contentment. When I was younger, I couldn't feel fully content if things weren't perfect; now I am way more laid back. I didn't try for this; it just seemed to happen, yet I must say I let it happen. I think there are those that fight it, see it as giving up or selling out and more power to them. Contentment is a very pervasive feeling, but not exciting or overwhelming as were other forms of happiness when I was younger.
A big part of acceptance, too, is knowing that I am going to die in a far more certain and far less dramatic way than when I was younger. I am not looking forward to it; I hope it waits for a very long time. But I know it is a certainty at a gut level, not just an intellectual awareness. It is way less scary than it use to be. I more feel a kind of regret for what I will miss. But I regard it now more like a necessary trip to the dentist or something similar. I don't want to go, and I wish I did not have to go so soon, but it will happen and then it will be over. It is almost impossible to explain the change in perspective.
So the short answer is the worst is decline in physical and mental powers and the best is acceptance of oneself and of the ways of the world.