Another late 50s runner here. Never really stopped, but had some years of injury setbacks along the way resulting in a runners equivalent of punctuated equilibrium/decline.
Peak : 25 to 34. I could get reasonably close to PR shape from the mile to half marathon with 2 or 3 months of base work and a few weeks of speed. 5K was the toughest to learn (took a long time to break through and didn't PR until 27-28) and it was the first to go. At 32 I didn't "have it" for the 5K anymore, but PRd at 8K and 15K at 32-33.
Decline: 35 to 39. Lots of life changes and stressors but I kept running while battling some nagging injuries. Put on 10lb which I never really lost. Performance dropped significantly (10K slowed >3 minutes from age 34 to 39).
Masters boost and crash: 40-45 Bounced back strong and had some good masters years; never approached PRs but ran ~13 sec slower in the mile, 1 min in 5K, 2:20 in 10K, and 10 min slower in marathon (compared to PR). For the latter I did significantly less mileage. Had more nagging injuries, which took forever to heal.
Comeback and cruise control: 46-55. Lost a lot of speed, but ran competitively for age group. Enjoyed some great and fun years. Had to adjust training with less speed work, more endurance training. Otherwise felt pretty good most of the time, as long as I incorporated recovery and periodized the training. Had very little decline in this period. Higher mileage than since mid-20s (~2700 - 3000 miles a year).
Decline?: 56-59. Recovery is more difficult. Speed is very hard to develop or maintain. No spring in legs. Mile and 5K times have dropped off a lot in 2 years. 8Ks are torture; 10Ks more aerobic and not too bad. Hanging in there for 15-25K; marathons not terrible but only do those every other year or so.