I don’t know if the age thing is the reason or not; my personal feeling is that it’s not.
And even if age is the prime factor; since we’re not going to change human nature, let’s not waste precious time and resources running up that road. Obviously, men and woman are not willing to settle and will continue searching for that non existent gold at the end of the rainbow, until it’s too late.
So why the age gap in the frum world now? Formalized Shidduchin.
Young frum people used to meet each other in informal settings and the age difference was minimal, or it didn’t make much difference since there was the hormonal factor.
And yes, many of today’s Rabbanim and Rebbitzens met this way too years ago; they would like to forget but I was there (Thirteenth Ave, on Friday night and Fourteenth Ave, Shabbos afternoon).
However, now we think that we can order what we want for shidduchin. Yes, that works if you have real yeshivish yichus or money; no age gap crisis there. But, for the “hamon am”: Get Real and don’t waste your time.
My advice: Marry any person who is a true Yiras Shamayim and that’s it! If you can do that, you’ve won and the rest is window dressing.