I can't remember precise times for every year, but here's my progression over the last 12 years in the mile, if that helps to assess age-related changes in performance. All of these at 4800' elevation.
41, 4:39
42, 4:34
43, 4:32
44, 4:39
45, 4:44
46, 4:41
47, 4:48
48, 4:46
49, 4:43
50, 4:42
51, 4:46
52, 4:51 so far (yesterday... went out way too fast in 2:19 trying to avoid being left in no man's land, and died very very hard.)
Based on a sample size of one, I feel:
40 is no big deal. Can still train hard and recover well.
45, hard training gets tougher. Takes noticeably longer to recover from really hard workouts, and something is always hurting.
50, slowing becomes completely inevitable. No amount of effort will produce the splits that were obtainable a few years before.