Nothing happens right away. It's insidious. Very gradual decline with maybe some horrible emergencies thrown in (detached retina for me). I'm 60 now and I can look back and see a significant decline since 50. At 50 I still considered myself somewhat sexy and good looking for my age, maybe a MILF even. Now there is no more denying it, I'm a troll, I'm OLD and look it. Things hang lower now, all things, everywhere. Things hurt all the time. Memory, hearing, vision, energy, strength, balance; all are decaying. At 50 most of those were still normal enough I didn't think about them. Now I think about them every day because the loss from what they used to be is becoming stark.
Ten years ago when I was 50 the world was still "mine". Now at 60 it seems the world has changed and left me behind. Culture has become unfamiliar and uncomfortable, I don't like it and I think the young people are doing it all wrong - and I recognize that is a typical old person complaint.
My thinking has slowed. It takes longer to analyze a problem and learning new things takes longer. But paradoxically, wisdom has increased massively. The amount of stuff I have figured out about life in the last ten years is probably equal to what I had figured out the entire first 50 years. But because expressing myself has become slower, and I frequently struggle to find the right word, it appears to young people I'm stupider, when in fact I am seeing vast swathes of reality all over the place to which I know they are blind. But all that wisdom is for naught; youth don't listen to me any more than I listened to wise old people when I was young. Everybody has to make their own mistakes and learn from experience. It's just excruciating to watch them do it.
My advice to someone turning 50 is to concentrate on two things. Plan for retirement, and do your big bucket list items. Do both simultaneously, though they might conflict financially. You don't want to put off either so you need to work them out together.