DY: the simple answer is that there are roughly the sme number of older single boys as girls, but the girls go to professional shadchanim and the boys do not. Therefore, the shadchanim think that there are more single girls. Also, the OTD ratio is a factor.
re: age gap statistics- it was demonstrated on other threads that the Avi Chai study AZ quotes reports only 2% growth. Multiplied by a 3 year differential gives 6% more 22yr old boys than 19 yr old girls. However, when you factor in the 5% more girls born, the age gap only acccounts for 1% unmarried!!! Also note that MO who marry similar age and have less children have the same problem, another nail in the “age gap” theory.