Your 50? Probably Just Age. Sorry Your Growing Old.Some reasons -not all-:
- Varicocele. http://en.wikipedia.org/wiki/Varicocele
- Cancer -hopefully it's not the case-.
- Testicle Torsion (but you wouldn't be here asking) http://en.wikipedia.org/wiki/Testicular_torsion.
- Not having sex for long time (nor masturbating).
- For young teenagers it can be related to the growth process.
Best of luck I hope it's nothing serious.