There are two elements in the shidduch crisis and they are often conflated.
The first is the overall imbalance, that there are [supposedly] more girls than boys, due to the age-gap (I am 100% sure that age-gap is a factor, but, as I have said before, there are many other factors at play, especially the ameliorating factor that more boys are born than are girls). My ultimate HUNCH is that only a small (2-3%) – though still horrifying – percentage of girls are victims of the age-gap issue (though I have no statistical data to support my hunch).
Then there is a second issue, and though it isn’t as problematic, in a way it is worse because it applies to a very large percentage of girls. If the average girl starts shidduchim at 19 and the average boy start shidduchim at 23, we are starting with a 4-year age gap. Ultimately, however, the average age gap between couples is smaller, say 2 years or 2.5 years. What that means is that an AVERAGE girl is forced to WAIT two ADDITIONAL years over and above the normal dating duration. Obviously, this additional waiting time isn’t uniformly distributed: some girls wait no additional time (i.e., the 19 year old girl who gets married to the 23 year old boy) and some girls wait 4 or more ADDITIONAL years. This is the only way to explain how we start with a 4 year age gap and end up with a two year age gap.
This is a massive problem. First, according to the SDEI CHEMED, delaying a girl against her will even a few months puts her in the “agunah” status. Second, when girls are forced to wait the ADDITIONAL two years, they are in absolute fear, worrying if they will ever get married. Sometimes, this additional wait time means that parents are forced to marry off the younger daughter before the older daughter, and are forced to dance and smile at the bittersweet (or perhaps it is better just to call it “bitter”) event of their younger daughter’s wedding.
Finally, there is a health issue. Without getting into specifics, every additional year that a girl delays her first pregnancy comes at a cost to her health. A woman body wasn’t designed to handle a shidduch crisis. This is a massive problem (google yenner maclah in the chest and in the colon along with “age of first pregnancy”).