I'm in my mid 50s, and I'm still here. I had the best job of my life in my mid 40s, and I kept up with every one of those 20 somethings with no problems. I was damn broke when I worked there, but was so happy because I loved what I was doing.
I, too, didn't think I'd make it to 30 and if I did, I'd be old and useless. Hasn't happened yet. Yes, there are physical issues with aging, but for me, there were also good things. I'm so much more self aware than I once was. I learned to stand up for what I believed in, regardless of personal cost. In my 20s I was so meek and a real pushover, now I won't take shit at all. All the years on this planet have given me the opportunity to see and do wonderful things, most of which cost nothing. So while body aches are not fun, it does not happen overnight. You have plenty of time before you have problems like that.
I most certainly understand not wanting to live to old age. For me, I view that is over 60... which is kind of funny because that's a few short years from now. I am hoping to die from natural causes in the next several years, as some of my biological ancestors did. I've lived the life I mostly wanted, have done everything I ever wanted to do, and have no goals or purpose left. I'm at peace with that and am mostly ready to go. I too will have to work until the day I die, and don't want to be 80 and be forced to be a Walmart greeter. I want to stay home and sleep. Although I will also say we have an 84 year old who works at our store and we can't get the man to retire. He WANTS to work.
A lot can change in a lifetime. Maybe give it some time and see what happens.