I've had bad birthdays and bad years, but they had nothing to do with age. As far as milestones my FAVORITE was 50. I was at my best physically and mentally and it was a great year. The mid 50's have not been so great but I'm always hopeful things will get better!
As time goes on I do feel sad that I'm sliding towards the end. And since I don't believe in the afterlife, it makes it all the harder knowing that I will just be gone and will never find out what happens with the world.
But I learn to enjoy life more all the time. I make sure I have lots of 'me' time, I do things I enjoy, I spend less time with toxic people (mostly family), I stop and breath and find joy in things often.