It is talking about young women and young men. So if a young woman dates a middle aged men, then you have a young woman that is not single. Since she is dating a middle aged men, she is not dating a young man. Woman can also date other women. So there you have other two young women that won't be dating a young man.

If you take a look at demographics, you'll see that at younger ages, there are more men than women. That the proportion swings the other way as they get older because men tend to die before women. So most single women are older, not young. Probably some single young men might stop being single by dating middle aged women, but since many prefer young women, they rather be single. That is OK, but as a result, they remain single. No big deal.