When do we get old

I'm four months away from 60. My body feels like I'm 80. Been walking around for 25+ years with some kind of undiagnosable arthritic type condition. Spent years trying to figure it out, and all the doctors could tell me is what it wasn't. It's not this, it's not that, etc. Finally just had to say the hell with it, and just endure it the best I can.
In my mind I'm still young, but like others mentioned, I do get young people holding the door for me. I always say thanks, but deep down it kinda pisses me off, like what, do they think I'm some kinda old man or something? Lol.
About eight years ago my mom passed in November, and then my last remaining uncle in December. This really hit me hard, cause I looked at the family tree and now me and my brothers are at the top. We are the old farts in the family. It's our generation that's going to start dying off now. I swear I walked around on eggshells for six months thinking that at any moment I was going to fall over dead. Took a while to shake that feeling, but doing better now.
But still think about the end a lot more often now. My dad only made it to 77, so I figure I got less than 20 years left, and I wonder things like, how many good years left? At what age did my dad have his first heart attack? How old when he got Parkinson's? Really hate thinking about this stuff, but it just pops into my head now and then.