For me:
Age 35 = first significant decline in results.
Age 39.7 = leg injury which caused me to be a much slower runnner overnight. (2:04 800m at 39.)
Age mid 40s = clawed my way back to 2:10 800.
Age late 40s/early 50s = steady decline due injuries / lack of good training.
Age 57 = quit running due to left knee osteo.
It happens. It's life. Some of us are lucky and run/train well forever.