Thanks for your query.Based on the facts that you have posted it appears that you have
noticed discharge from your penis on sexual thoughts.
This is not abnormal at this age and is due to excessive secretion of the peri urethral glands located in urethra.
As regards the size of the testis there is no defined physiological standard of the size of the testisi n relation to the age .It defers from person to person.
If you have any doubt about the size please consult qualified Urologist
for clinical evaluation and further treatment if required.