In Australia, for example, “almost twice as many boys as girls had a disability (192,800 or 9.6% of boys compared to 103,600 or 5.4% of girls). This sex difference also applies when we consider the pattern for children with a severe disability (97,400 or 4.9% of boys compared to 47,000 or 2.5% of girls).”
Regarding the OTD rate, I spoke to a statistician who studies the shidduch crisis, who stated that he believes that the greatest factor in the shidduch imbalance, even more than the age gap, is the OTD-rate differential (if I remember correctly).
The OTD-rate differential, the early-death-rate differential, the disability differential–we can’t control. We can control the age gap, however, and we can FACTOR in all the other differentials to produce the optimal age gap. It isn’t a perfect science. Perhaps the optimal age gap is for girls to be a year older; perhaps it is for the boys to be a year older. We know two things for certain: a) shadchanim say that have many more girls; b) we seem completely OBLIVIOUS to these issues when deciding when children should start shidduchim.