Yes, semen does decrease with age.
This can occur both due to an actual decrease in production and more likely due to decrease in ejaculatory force.
Decreased ejaculatory force is often a normal temporary phase which passes off soon.
As we grow older, the ejaculatory force also decreases naturally.
Some other conditions which could cause such weakening are increased sex, too infrequent sex, (two or three times a week is ideal), stress, inadequate sexual interest in partner, etc.
So simply from decreased semen, it is not at all possible to make a judgement of this nature as it could also very likely be due simply to increased age.
The best way is to maintain a healthy lifestyle, by avoiding smoking specially, having a low cholesterol and exercising regularly. All these steps help to keep the circulation normal and help in ejaculation. Taking adequate fluids so that the person is well hydrated is also necessary. Kegel's exercises will also help the person to have a strong orgasm and ejaculation.
Three supplements that are believed to help in this are Zinc Oxide, Arginine and Lysine.
You can look up here: http://www.articlealley.com/article_176755_23.html
For Kegel.s exercises: Link
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