Multiple births occur in most mammals, with varying frequencies. In humans, multiple births are relatively uncommon (aside from fertility treatments). But the incidence tends to rise with the increasing age of the mother - although fertility itself declines. This tendency is unexplained.
In studies performed before the inception of ARTs [assisted reproductive technologies] and controlling for parity, twinning rates were found to increase by 300% between the ages of 15 and 37 years (Bulmer, 1970). This increase is particularly prominent in the maternal age group of 35–39 (Bortolus et al., 1999). No scientifically based explanation for this phenomenon has been forthcoming. It is all the more fascinating as it occurs, paradoxically, in the age group when fertility potential starts to decline rapidly."
Source : Human Reproduction, Volume 21, Issue 6,