As can be seen from the following link, an young adult male can have a serum testosterone (T) level in the range of 300 to 950 nanogram per deciliter of blood. :
This range also slightly varies among different labs. The broad range is to cover a larger population base. It is is also expected that with age the T level reduces by about 1% per year after the age of 30 years. Naturally it implies that at any age there will be an acceptable range for the T level and there can not be a fixed value for a given age. Also it means that if a person had a T level of of 900 at his youthful age, and at sixty years if his T level drops to 400 he could develop symptoms of low T even though he is within the normal range. At the same time, a person who had a T level of 500 when young, could be symptom free at 400 level and 60 sixty years of age. However, at any age, T levels below 200 is a definite indication of hypogonadism.
Here is a discussion on age related treatment of low T and its effectiveness:
Thank you very much for taking the time to answer my question. It is difficult to know what your T level was when I was young.