Even so, let me throw out an "average" testosterone level table by age to start out with simply by starting with a peak of 700 and decreasing by 50 ng/dl each decade:
Age Tot. Test.
25 700
35 650
45 600
55 550
65 500
75 450
85 400
As a verification, one can look at this link and find a normal or average testosterone level chart by age. [3]
Avg Age Tot. Test.
30 617
40 668
50 606
60 562
70 524
80 471
93 376
If you do enough research, you will find other similar results and average values can indeed get a good feel for the age-related decline in testosterone levels.
Now let's jump to some important points when comparing your numbers to these kind of average or "normal" male testosterone levels. First of all, you must remember not to panic if your testosterone is a little below what is given above. For example, there are many guys with testosterone in the 400's that are doing great with solid libido, erections and heart health. In fact, guys like this probably have no idea that their testosterone is below "normal" and are doing just fine.
Although let me jump in and say that many young guys have written in that are struggling with testosterone levels in the 400's as I mentioned. Young males seem to be particularly sensitive or perhaps it's the relative value compared to age that really counts. Regardless, I have known several younger guys who have reported dramatically improved erections and libido when given supplemental testosterone even though they were in the 400's and were supposely "normal".
There is something else you should keep in mind if your testosterone reading is below the average values mentioned above. If you are eating a Low Fat, Ornish, vegan or vegetarian diet, your testosterone levels may be lower than the typical male omnivore. (I cover this in my link on The Two F's of Testosterone.) However, this is not, in general, a bad thing as vegetarians, for example, have excellent health outcomes. You will for example, likely have solid erections on a Low Fat Diet because it boost nitric oxide levels so powerfully even though your testosterone levels are lower than a meat eater.
So if your testosterone level is below average, regardless of whether you as a male are experiencing classic Low Testosterone Symptoms, you may want to take corrective action. I cover many dietary and lifestyle factors on my site that can naturally boost testosterone. Read this link, for example, on How to Boost Your Testosterone Naturally. And, yes, that even includes Sex! It may be possible to boost your testosterone levels 20-40%, and possibly get them into the normal and acceptable range, simply by changing certain lifestyle factors.
REFERENCES:
1. J of Behavioral Med, 1999, 22(1):1-19
2. J of Clin Endocrinology Metabolism, 2007, 92(1):196-202
3. Vermeulen A. Declining androgens with age - an overview, In: Androgens and the ageing male. Eds. Oddens B. Vermeulen A. Parthenon Publishing. New York. 1996