Personally I think perhaps early 40's for a sharp decline.
My trigger was the op which sent everything tumbling quickly so I really put it down to that and the major depression I suffered following that with all the anxiety,stress, lack of sleep, poor diet, no will to live, socalise or anything else...Testosterone cannot survive in that environment..no way
My terrible diet and regular drinking would not have been helping my T levels previous to that despite the positives of excercising and feeling generally good. But I had no real symptoms then and would say that I probably had a normal slow decline not helped by some unhealthy habits.
I think FightlowT, symptoms will manifest extremely quickly following depression and as you know it is a vicious circle of not feeling good which contributes to more of the same..and from then on in its a major battle to climb back up that happy hill..
I think I was probably in the 500's plus and due to circumstances dropped to the low/ mid 300's and thats when I felt it...Now I am probably around the mid 400's if I was to judge on how I am feeling at present..but thats just a guess..I really need to get tested..