Why Is 30 Considered "Old"?

I turned 30 last September. I had no problem with it, but everyone else around me did (meaning my older sister, brothers, cousins and friends). As more and more of my younger friends turning 30, it keeps coming up all the time. I heard stuff from everyone and it led to many different questions and conversations..

"30 was hard for me to accept." Why? "I don't know. It just was."
"I should have been married with kids by 30." Why would you put a timeline on that and not just let it happen naturally?
"Next comes 40." Yeah, but I have 10 years of living to do before then and I'm sure I will be just as accepting of 40 and I was of 30..
"You're old now." Let's put aside the arthritis I have had in my joints since I was 23.. I have always been called old-school or an old soul.
"You're not young anymore." Am I supposed to go berserk now and get all midlife-crisis?

Apparently, it is an unwritten rule that 30 is supposed to hit you like a ton of bricks? I think I missed the memo about that one. I don't get the hype. People called and texted me all day asking how I felt. Umm fine? No different than I did yesterday. Getting older is a part of life, no? Why is it such a shock that 30 comes after 29? From one day to the next, you go from young to old? That makes no sense at all to me. 
To be clear: Am I where I thought I would be by 30? Absolutely not, but that's okay. I realize it was silly to put a timeline on when I would accomplish certain goals. I will get to my bucket list when I get to it. No need to get all depressed about it.

"You're only as old as you feel."
When people hear that I'm okay with being 30, they then tell me I'm only as old as I feel. I have a problem with that because if it goes by feeling, I am all different ages. Some mornings I wake up and am eager to start my day. Other mornings it rains and my arthritis prevents me from standing up straight and pain-free right away.

I guess everyone has their reasons and the age they consider 'old'. I just don't think 30 was it. To me, I would say 70 is old. 30? Not so much.