At what age do your testosterone levels sink?

Any Age. I am finding 20 years olds with low testosterone. It used to be that i did not see this until a man was 50 or older but i now test any man that is having fatigue and difficulty getting and maintaining erections or loss of libido.
The Early 30's. See the following graph: http://img403.Imageshack.Us/img403/8324/atlbaul6.Jpg in order to know your level, you must check a morning total testosterone level. If it is less than 300 ng/dl, then you may have low-t.