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A new study has cleared up the conflicting evidence about the extent to which men's semen quality decline with age, likely lowering their fertility. 

According to the study, there are consistent age-related declines in semen volume and sperm performance and increases in malformed and DNA-damaged sperm. Semen quality is considered as a proxy for how fertile a male is. 

Authors of the study said understanding how age affects fertility is becoming increasingly important as couples delay childbearing toward later stages of their lives. 

"While female age is well known to have negative effects on fertility, reproductive success and the health of offspring, the influence of male age on a couple's fertility has been largely neglected," said lead author Dr Sheri Johnson in the press release. 

"The effects of declining semen traits with increasing male age have largely been ignored due to inconsistencies in the literature, but our work now suggests that male age affects a variety of traits. It is well recognized that reduced sperm performance can affect pregnancy success, but it is less well known that the quality of the sperm, particularly DNA quality, could affect the development and health of the offspring." 

Researchers synthesized the current state of knowledge from 90 individual studies, spanning around 94,000 volunteers/patients from more than 30 countries. 

"Our study made no attempt to estimate the rate of decline, but some well-controlled clinic-based studies have observed consistent declines with increasing age, whereas others project declines after age 35 for some traits and after age 40 for others" Johnson added. 

The study will be published in the journal Aging Research Reviews.