My knees have been going out since I was like 16, so I’ve felt pretty old for a long time. I think my biggest old person thing is being very particular about my bedtime each night and always getting up at the same time. I definitely did not do that in my twenties.
Testosterone got the better of me weight lifting in high school and I fucked up the ligament that holds my left kneecap down.
I pop and crack like a 70 year old and I'm 29 (been happening since I was 17) and I can't keep my left knee bent for long periods of time without it aching.
I'm trying to stay young by playing video games (primarily rocket league) but they don't feel the same as when I was younger. That was my actual sign.