It's true that I talked like that (although I think I was saying 40 was when the warranty ran out), I have come around.
Starting around age 50 I augmented my exercise routine with a tai chi practice, then natural movement, and found that instead of gradually wearing out, I felt better every year. By age 60 I felt better than I'd felt since I was in my twenties.
I'm no longer making progress each year, but I'm no longer declining (...so far, at age 67).