… when you begin an anecdote and realize midway through it that you’re going to have to explain a handful of things to the younger listeners in order for them to understand your tale.
…when smart adults on Jeopardy! are too young to put things in historical order… things that you remember in your own lifetime.
…when your own parents, if they were still alive, would be over 100 years old.
…when you consume more pills than food.
…when your kids look middle-aged.
And you know you’ve gotten old when every part of you has either dried up or leaks.
The “100 years ago” thing is starting to get to me. Not just that so-and-so from my parent’s generation was born 100 years ago. But that people from my grandparent’s generation died 100 years ago.
E.g., my grandmother lost two siblings, young adults, to maybe the Spanish Flu nearly 100 years ago. My grandmother died not that long ago. Well, not terribly long ago.
A century is a long freakin’ time. And now I’m thinking about stuff that happened that long ago.
I distinctly remember the first time I felt old – when my library received the new American Girls historical fiction series. Set in 1974, the year I graduated from High School.
When the standard form of respect from strangers is “Thank You, Mama.” WHEN did I become a Mama?
Or they see you as “safe” and they talk to you about things like boy friend troubles or dating.
(That’s not a complaint btw)
When most of your favorite movies are black-and-white… or silent.
When you remember when it cost 3 cents to mail a letter, and a nickel to make a phone call.
When you can neither bend over nor stand up straight.
I’m 57 and when I’m not biking to work, with a 30 km detour on the way home, or walking the 6 km to work with intervals of jogging, I often get around by bus or metro and have been offered a seat a few times.
So I went to the doctor the other day, and while I was killing time waiting for the doctor, I measured how tall I was with the how-tall-are-you thingee. Now, I have always been 5’9" and a half, though for official purposes, I rounded it to 5’9". So what do I discovered in the exam room that day? I’m 5’7" and three quarters. At some point in the last 20 years I have shrunk by almost two inches! What’s up with that??
Age 4: Success is not pooping in your pants.
Age 12: Success is having friends.
Age 16: Success is having a driver’s license.
Age 20: Success is having sex.
Age 35: Success is having money.
Age 50: Success is having money.
Age 60: Success is having sex.
Age 70: Success is having a driver’s license.
Age 75: Success is having friends.
Age 80: Success is not pooping in your pants.
…almost everything hurts and what doesn’t hurt, doesn’t work right.
…the names in your little black book all end with M. D.
You realize there are people who voted for Hillary Clinton who don’t remember when her husband was President.
You have more doctors than friends.
…you read this thread so you can chuckle at the old farts on the Dope, then realize it all applies to you too. 
Me, too. My paternal grandparents lost siblings in the 'flu epidemic.
Movies and music is where I feel it, and usually when I compare them to the things I thought were old when I was enjoying them.
Like when I realize that Nirvana is as old to today’s high schoolers as The Monkees were to me.
Or that Ferris Bueller is as old now as The Ten Commandments was when Ferris Bueller came out.
My favorite toothpaste, moisturizer, and bra were discontinued last year.
I have related this in other threads but when the Orthopod told me my knee would never be totally pain/problem free again, I couldn’t have cared less. When the Cardio-guy said I would have to carry nitro the rest of my life, no big deal. When that eye doctor looked at me and said “Mr Kopek, there is no way to avoid it any longer — you HAVE yo have bifocals” I was crushed. I mean I seriously wanted to crawl out of there. I was finally old.
Today I was listening to some old Anthrax records, when it struck me that their album Spreading the Disease was released in 1985 – more than 30 years ago. Which means it’s closer in time to Bill Haley’s “Rock Around the Clock” than it is to today.
Or they show a picture of someone like JFK and the contestants are supposed to guess who he is.