"Am I too old to do a 540 kick? A backflip? A b-twist? etc?"
Is it age that is holding you back? Why age? Just because your knees ache doesn't mean you're too old, it might mean your legs need strengthened. It might mean your knee cartilage is a mess from abuse (or disuse, have you used your knees in awhile?) Have you been sitting in a chair too long? Are you overweight? Can you do something about it? I bet you can. Eliminate the "too old" excuse from your vocabulary.
"Do I have the physical potential to do a 540 kick? A backflip? A b-twist? etc?"
Why wouldn't you be capable of learning a particular trick? Do you have a real disability? If you picked just one simple trick to start with, could you nail it if you really put in the time (months) and effort (daily practice) to learn it? And if you could learn any one particular trick, what's stopping you from learning a couple of other tricks too?
I didn't wonder if I could be a good trickster when I started tricking, I didn't even think about being a trickster, all I wanted was the 540 kick! That's all I cared about. Six months of busting my ass and one new 540 kick later, I found myself taking an interest in other tricks...
A recent example
At the time of writing this I'm 23 and have started playing the piano this year. I'm not good at it, and I'll never be as good as those kids who've been playing since they were 5 years old, but I don't care! I didn't start playing the piano to be a concert pianist, I started because it seemed fun, and because I thought I'd be cool if I could play a few favorite songs.
Hey, if my electronic piano explodes tomorrow and I decide not to get it replaced, would it have been a waste of my time? No way! So what... I started something and then quit and never got good at it: Big deal. If you decide to quit tricking after a year of practice would you really regret having spent time on it? No way! You'd be awesome for putting even that much time into it.
Just try it! Yeah! Come on!
When you enjoy what you do, you'll figure out a way to be excellent at it. Go out and see if you enjoy tricking, give it an honest attempt, a few tries. If you don't like it, that's okay, try something else. You've got nothing to lose!