From my observations, I'd probably bump this up to when a woman is starting to approach 30 and beyond. Most women that are in their early 20s are often still all about having fun, partying, etc. However, I've noticed that this seems to settle down by the time they get into their mid 20s and at this point are often starting to settle into longer term type relationships. If a woman is still single by her late 20s the energy that I've noticed is that they are starting to feel their biological clock ticking and that they are looking to get wifed up soon. For a woman to still be single by the time she hits 30 there are reasons for it and none of them are good.
One thing that I believe is a very serious and quite obvious red flag is if a woman is attractive or possibly even hot and yet is chronically single or a serial dater once she is past 25. Most men will try to lock down such a woman into a relationship fairly readily, so for one like this to be single shows that there is something going on with her that is causing these men to run to the hills.
Something else I've seen over the years is career/ambitious women are often times single, I can think of several women that I know of who remained single into their 40s. Based upon what I saw, I felt that the reason for this was simply these women were just not at all available to men, both emotionally and physically. These women are operating on a masculine wave length and many men are genuinely turned off by this and will quickly lose interest. This is fairly common sense and doesn't really need to be elaborated on.
Now for women that are neither of the two things above there still is something going on. Even very unattractive women will be able to land relationships, tons of men have virtually no standards and will go for anything available. These variables at play could range from social issues to perhaps having a super obnoxious personality or even being over picky beyond their actual league. I do feel that many women today view themselves higher than their actual league and men as lower than they truly are but even women like this can land men.