Problems such as chronic illness (mumps in childhood which damaged testis), aging (not in your case), starvation, head injury, surgeries in testicular area, cancers, infections, and alcoholism are also among the causes of low testosterone levels
. Please consult an endocrine specialist and an urologist.
It is difficult to comment beyond this at this stage. Do consult your doctor ie PC nad go for proper referrals.
Hope this helps.