Absolutely agree OP. I can literally see an egg timer with the sand running out.
I think for me it's watching the effects of ageing on my physical body to realise I am now a 'mature' woman. It does not bother me really but I do feel shocked as to how I got here. I mean I know everyone ages but somehow thought I would be exempt.
Then my parents both dying. Before that someone else was before me in the queue (for dying). Now they are gone I am pushed up to the front and it's very much a feeling of disbelief that they are gone and that 'I'm next'
Finding out my first serious boyfriend had died at 50 from cancer. So the tall, charming boy who I adored and loved is dead. That kind of knocked the wind out of me and made me realise time was marching on.
Even watching neighbours kids who were small toddlers when I moved to my house and now they are on the cusp of being young woman heading to high school feels shocking and really emphaises the passing of time. I don't notice it nearly as much with grown ups but kids changing really makes you aware that time is rushing by.
I'm retired now (early fifties) and whilst I am very happy to be out of the rat race saying 'I'm retired' outloud to anyone just seems surreal. I remember so clearly being the twenty something in the workplace and being a bit dismissive (in my head, not externally) to the 'old guys' in their fifties. Now I'm the 'old guy' the twenty somethings are dismissing.
Also as you say regret over decisions I can now see were bad ones and took me down a path which was not so good. Good men I didn't appreciate when I had them, houses I sold that I shouldn't have, things I worried about that were not worth my time.
The plus thing is that I worry less about things that used to concern me a great deal and now seem utterly ludicrous. I used to be houseproud, now I'm pretty untidy. I used to worry about my weight/appearance, now I'm way less concerned about it. My attitude is very much that only serious illness or death are worth worrying about to any degree. Everything else is just 'noise'
I've also developed an interest in lots of things I never use to be remotely interested in such as art, growing stuff, documentaries on planets, space, world events, world wars. I also have animals now and far favour them over buying 'stuff'.
Sometimes I think about how I want to die. My mum lingered for ages and it was not pretty at all. I hope for a quick death at a decent age. Early eighties will be fine. A quick heart attack in my bed or chair and no long hospital stays. I also don't want to leave any animals behind that have to get rehomed. Apart from that I am gradually making my peace with it the older I get.