It seems well known in contemporary society that it becomes harder for a woman to be impregnated as she ages - that is, maternal age is a factor in female fertility.
A 2003 study in Human Reproduction concluded "semen volume and sperm motility decreased continuously between 22-80 years of age, with no evidence of a threshold." It appears male sexual function diminishes over time, semen volume decreases by 0.03 ml per year according to the same study, stopping before the point of infertility.
Human Reproduction is the official journal of the European Society of Human Reproduction and Embryology, published by Oxford University Press. The study can be found with the following academic citation.
B. Eskenazi , A.J. Wyrobek , E. Sloter , S.A. Kidd , L. Moore , S. Young , and D. Moore. "The association of age and semen quality in healthy men". Human Reproduction, 18: 447-454. European Society of Human Reproduction and Embryology.
K.K. Ng , R. Donat , L. Chan , A. Lalak , I. Di Pierro , and D.J. Handelsman (2004). "Sperm output of older men". Human Reproduction, 19: 1811-1815. Advance Access published on August 1, 2004, DOI 10.1093/humrep/deh315. European Society of Human Reproduction and Embryology.