I'm 48, almost 49 and just recently I feel like the realisation of my advancing age is really starting to hit me.
I now need reading glasses but I wear multifocal contact lenses (I'm also short sighted) so no-one knows my 'secret' apart from my optometrist. 
During my 40s, I fell into bad habits after a traumatic relationship breakdown and job change. I stopped exercising and instead I comfort ate. I arrived at age 48 grossly overweight (by 5 stone!), in bad shape and miserable (and disgusted with myself). I used to be superfit and slim and the 48 year old looking back at me in the mirror was unrecognisable. I looked like a fat middle aged woman. 
So I got my shit together, got a hardcore personal trainer and got my fat arse down the gym. That was just over 7 months ago. I've now lost almost all the 5 stone (4 pounds to go) and I'm much fitter. Everyone tells me how great I look. It had given me my confidence back but as I approach 49, the penny is dropping that although I look good for my age, I am a middle aged woman still. I'm old.
I did a HIIT class last Sun morning and was talking to the super-fit female instructor afterwards about how getting older sucked and how I've become invisible to men. In this case it was triggered because I was ignored during the class by the two men in my team when we were planning rowing strategy (team that did the greatest distance in the time won but we also had to do other exercises so strategy was key). I didn't bother arguing back but I felt frustrated at my opinion being ignored (especially because they didn't realise that I use to row competitively internationally so I probably knew more about rowing than the entire class put together). It made me wonder if they would have paid attention if I'm been a 20 something rather than a 40 something. I felt like I was blanked because I was a middle aged woman. Anyway, I was telling the instructor this and she said to me "I hope I'm as fit as you are when I'm your age". Talk about a back-handed compliment. It made me realise that by late 40s, there is no getting away with not looking middle aged. You can look young for your age but you just look a younger middle aged rather than younger if that makes sense. My personal trainer is amazing but he can't work miracles and turn the clock back. It's a sad realisation and I wish I could have carried on fooling myself that I'm not that old.
I also worry about my father. He is my only close living relative and the one person I speak to on a regular basis. He is 77 and I worry about how badly I will handle it if/when something happens to him. I think I will fall apart and not want to carry on. But then I started worrying about what will happen if I die when I'm still alone. Who will clear my home? What will happen to my things? Will I be left for days before I'm found? I promise you all that I'm not usually this morbid/miserable but this realisation of mortality and being alone has hit me hard this January. Perhaps because I'm in a bad place anyway (my job was cut in Dec and the job market is dead so I'm living off my savings and worrying so much about the future - like so many people are).
I am trying hard to convince myself that 50 is the new 40. But in these dark times (for the UK with Brexit) and in the depths of winter, it's a hard sell.