When did you realize you were old?
Me: When I would rather stay in on the weekends than going out.
When my desire to not have a hangover or a DUI compelled me to almost always avoid drinking.
Menopause and it’s lovely symptoms.
When I look in the mirror and wonder “How the hell did that happen—and so quickly?”
When having fewer wrinkles and not wanting skin cancer became more important than being tan.
When being comfortable and taking care of my feet and hips outweighs choosing 3-4 inch heels.
Some of the things I notice might be more an effect of maturity as opposed to feeling or being past the middle point.
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.
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!