It's not linear and everyone is going have their own path. It's going to come down to injuries, motivation to train, weight, general health, genetics (mr. it's all genetics you can chime in here!) and lifestyle and no doubt other things.
I did a quick review of my own decline from my peak some +/-35 years ago and selected 5k as model. If you go by the common rule of thumb of 10% decline a decade after 35 or 40 (ran my PR well before that age) I would have lost some 25% or between 3.5 and 4 minutes. And if you look at age grading, I would have lost about 23% (3:40).
Here some data, N of 1, looking at my best for each following 5 year age group
PR at 28 - call that 0%
30-35: -2.3%
35-39: -7.2% (my biggest drop, fighting injuries most of the time)
40-44: -7.2% (same time as 35-39)
45-49: -12.2% (coming off a 3 year bout of chronic injury)
50-54: -11.8% (ran a couple seconds faster than previous age group)
55-59: -13.8%
60-62: -14.8%
Some will lose less. Others more. I'm actually pretty lucky. I do think I could have run better from 35-45 but only had 2.5 healthy years in that span (at 40-42), and it was also a time when I had a lot going on with career, switching a few jobs and moving across the continent a few times, young family and all that. However, have been extremely lucky from my late 40s on, and in particular in the past 5-6 years when I haven't dropped off much (only lost 10 seconds in 6 years for the 5K).