It's probably all normal...do not worry. You should remember that inspite of an ego problem concerning low semen quantity, that orgasms still feel the same. However, repeat performance may suffer.
As a male gets older, there is a marked decrease of the male hormone, testosterone. Testosterone is the most important of the male sex hormones. It is responsible for stimulating bone and muscle growth, and for sexual development. It is produced by the testes.
There are testosterone patches and pills to take mainly to increase sex drive and ability. See your Doctor.
And there are natural herb testosterone-type supplements available at your local health store like Stamanex Male Potency Formula, and Avela. Read labels carefully for side effects.