I think the problem is that it isn't just the number... it is when did you get the injury that never really went away? If that happened at forty, then the answer is forty. After each one of those injuries, there is a "dropping down a level" that you can never recover from. Pulling your hamstring at age 47 means that hamstring will never really be 100% again.
The second thing is metabolic/hormonal. At some point you will notice a drop in T that causes the burning of calories (at rest) to slow dramatically without lowering the appetite. This means if you eat the same thing at age 50 that you ate at age 20 (even if you are running more mileage) you will gain weight. That means you have to eat less even you feel just as hungry as before.
So yeah, getting old is a b!tch. But I was always slow, so "getting slower" wasn't as life changing. But here is the break-down (roughly):
20 - always getting faster each race or each season
25 - peak 5km ability and ability to recover from workout to workout
30 - no longer PRing in the 5km so I gave it up and tried a marathon
35 - hated that training wasn't making me faster anymore; switch to cycling
40 - missed running so much, used Masters Running as motivation to run again; set all my Master PRs
45 - ran every day without injury but still slow as hell; tried some ultra/trail runs
50 - everything hurts, can't fully recover from injuries, but still running every day though I have no speed whatsoever