What makes you feel young? Kids don't have to worry about this -- they just are young, of course. Young adults don't have to think about it much, either. They are still young. And it seems to me that young adulthood gets longer and longer, and that is a good thing. My granddaughter expects that I will live to be 100, and that would give me another 28 years to the finish line. 
When you are young, 28 years seems like a very long time. For me, at this point in my life, time is ever-changing. Some days it seems like there is a lot of it. Some days just whiz right past. I spend too much time thinking about what I should be doing when it seems there is too much of it, and too much time thinking about what I should be doing when it whizzes by. You get my point? This seems to me to take up too much of the commodity I'm trying to savor.
As you move past that ever-changing demarkation of what passes for "middle" age, there are some things that will push themselves in, whether you want them to or not. And if you have not had much body-consciousness before, you will definitely have more of it now. Knees may not support you reliably. When you want to kneel down to give your grandkids a bath, your knees may let you know that this is NOT something they are used to doing. And when you try to stand up again, you may need to use your arms to help your knees support your weight. Unless, of course, you are one of those fit people who jog or practice yoga every day. Then you may go right on as you have before. I am not one of those people. And I find some resentment eating into my time along with the 'what I should be doing' mantra.
Information is good, though, so if wise, you will begin to take better care of joints, exercise more, practice balance, eat better, watch your weight, all of that body-consciousness stuff that may not come easily because up until now you have not really had to think about it much.
Gray or graying hair can put a damper on your enthusiasm for aging -- unless you are lucky enough to have a wonderful mane of silver that miraculously appeared almost overnight. Then it can be stunning. I am not one of those lucky people, my hair started going gray in my fifties and twenty years later it still is a big mix-up. When I get it trimmed, I see more gray on the floor, but there is still a lot of dark brown too. Of course I have tried the coloring touted so loudly to even very young people who need to 'cover the gray,' but it never worked for me. Too much trouble.
Women do spend a lot of time worrying about their hair, though, and bad hair days can really put you in a funk, still, even though you are out of adolescence. Nothing can give you a bad body image quicker than a bad hair day, even if you have just lost 20 pounds.
Clothes get trickier too. If I wear this will it look like I'm trying to be too young? If I choose that, does it look too matronly? Is that color too daring? Is that cut too low? More time wasted trying to do what? Stop time? Fit in? Not fit in? Phooey.
My hairdresser caught me on a day when I was feeling pretty good - no aches or pains - knees working fairly well - good energy level. She had just put some colored 'lights' in her hair, and it looked like so much fun! She asked me if I wanted some too and I thought, "Could I? What would people think? Do I care what people think? Well, yes, of course I care. But might I try it anyway?"
In the end I had her put some blue 'light' in my hair. 
The responses have been mixed, but you know what? I'm having fun! And I have discovered that when you are having fun, you don't worry about time much at all. Let time do what it will, I'll get some fun out of it and feel younger every time I do. I suspect the more fun you get out of living, the more time you get. No, really!
