I am 40 and I just got my test level done....it's 550! GREAT! My doc said that their are 60 and 70 year olds with normal test levels and that not all men experience a decline affter 40-45. He told me that his test level was 575 last time he checked and he is 53!
I think one reason, other than youth, that young guys gain better than guys over 35 is because the very young are nowhere near there maximum natural size(in general) Many of us old guys He he he, that have been training for many years are nearly maxed out and then, of course, gains will be slower.