There is a popular opinion in the Orthodox Jewish world that there is such a thing nowadays as a "Shidduch (Marriage) Crisis", and that a major contributor to it is something known as the "Age Gap."
This theory has been repeated many times in printed and online sources, including this Five Towns Jewish Times "Shidduch Island" article and this YouTube video which was apparently presented at the 2009 Agudah convention.
The idea is that since there is population growth in the Orthodox community (many families have 5-10 children in addition to Baalei Teshuva/Repetents and since men get married older (by at least 4-5 years) than women, there are more women than men at a marriageable age. This results in some girls getting stuck in a "musical chairs" scenario (called the Age Gap Theory), and are unable to find a boyfriend.
If the difference in marriageable age would be 40/50 years I could see this theory to be true.
On the other hand, other (like Chananya Weismann) write that an age difference of 3-5 years shouldn't result in a statistically significant amount of women "left behind" due to the Shidduch Crisis.
Who is right? Are there more 23-year-old Orthodox Jewish men than 18-year-old Orthodox Jewish women, and does this lead some women to be left without an eligible match within the community?