As I read your question it occurred to me.

Oh shit! I am old.

Thanks for that.

When hair started growing out of my ears

never, dear, never.....

when i did the pogo for 2 hours and couldn't walk properly the next week [ last week after orchestral manuvers in the dark]

When I went out and tried to pick up a 20ish young woman and she give me the look plus a EeWW! ?

I realized it when I walked passed a cemetery and two guys started chasing me with shovels.

Old? I still think I am a kid, though my body and some othe prople point out otherwise. I have a 9 year old daughter who is too often mistaken as my grand daughter. I was at the park with her and another parent with her kids and additedly she looks young next to me. But another little kid asked is she could play, being a parent she tried to decline and the kid said "you could ask your dad, pointing at me"
THAT is when I realized I was old.

It is as I had suspected, I have been old for many decades. How does this happen?

My kids make me feel old a lot but that's normal. One thing that makes me feel old without their influence is singing along to grocery store music. I was in Wal-Mart the other day and heard Anne Murray and I was so excited because I hadn't heard her for so long, I forgot she even existed!

I agree when staying home and watching Netflix sounds better then going out. But the biggest thing is now I grown a little when I get up off the floor from working on a electrical outlet.

By your measure, I was old back in college.

I looked...you are only 45 years old....that is not old. Wait until you are Medicare, then you might be old.

When you realize the Music is way too Loud!~~~~

The people around you look like they could be your daughter or son!

Staying home more than going out, even on the weekends!**

LOL!

When I noticed how young all police officers look!

I'm 51 and filing for disability.

When I got diagnosed with a life-threatening disease like leukemia. Most of my friends/exes have been younger than me.

what do you mean ... OLD ?
Try as hard as you like , you won't catch me up.

When I noticed that the majority of the people who were adults when I was a small child are now dead.

When it took two shoulder surgeries just to keep working out, 10+ years ago. =[

I'm old? OK, OK, I'm old. I think it was kind of the same thing someone else mentioned below; jumping down from even relatively low heights became daunting, running turned into very cautious loping, climbing ladders is scary, routine things that I didn't much think about doing when I wasn't "old." Then there was the realization a few years back that the little baby girl I used to cradle in my arms was a grown woman with a whole life of her own.

I'll add something to an earlier answer. I resist the idea that I'm getting old but it is getting increasingly difficult when my 11 year old groups dinosaurs, the stone age and me and being around at the same time.......Love you

...when I realized I was more mature than my father.

When my 17 year old nephew saw a female friend that is in his college class and told me to go ask her out because she is nice and she an older woman. When I told him that she was too young for me, he replied no she is not, she is older than me and starting college late, she is 23. I pointed out to him that I am more than twice her age. His answer was; " oh yea I keep forgetting that you are old."

When I started thinking home appliances were fun to look at.

When I had to have organs removed and body parts repaired.