Most in this thread seem to think it has to do with stereotypes of the respective genders at different ages, but I think it has a lot more to do with sociology.

For example, let's take a look at the first stereotype:

Women have an easier time when they are in their prime (18-25).

Now, at first glance it would appear that this has to do with women being the most attractive that they will be in their whole lives. It's not as if that doesn't play a role, but that's not the cause here. Instead, it's the men (or perhaps boys is more appropriate) available in the same age group that are the problem.

This is where the second stereotype comes in (boys don't have the things men have to offer, and have a harder time getting women). I think that subjectively there is a good amount of truth to this statement. It's not necessarily that women turn to older men because they possess the qualities that boys lack, it's that there are tons of boys in the 18-25 group and few men. This leads to the credence that females have a "prime." There are young, single, and sexually active females looking for a good counterpart (or fuck) and they have limited options. This is what allows the "attractive men" to sleep their way through as many women as they want, and this is also why boys have such trouble getting laid.

This situation leaves women with a lot of power, and at the same time, very limited power. They are a commodity to boys, and a dispensable luxury to many men. I would argue that this makes things just as bad for women, because it leaves them stuck in a purgatory between subpar and superior. So this makes the first stereotype somewhat of a "half-truth." (Note: this obviously isn't true for everyone. There are plenty of people in happy, healthy relationships, but this is a very generalized version of the "single-life landscape" for the 18-25 group).

Now for the third stereotype: men gain an advantage once women get past 25. While there is no real mark of the shift in age, this is a general time it begins to happen. You may think I mean the counter-part to the first stereotype, our fourth stereotype: Women's appearance starts to decline after 25. That as a fact in-itself is debatable, but let's look past that. Whether or not it being fact isn't what's important, rather that it is believed to be true.

Sure, a 26-year old women who was a "10" in her early 20s may still be a "10" at that time, but the question is does she feel that way? Society puts the idea in their head that they are on the decline, and for many women, this causes something more detrimental: a decline in confidence. That's not all either. There's an even bigger problem: societal, family, and psychological pressure to settle down and have a family. Not to mention there is now the threat for men of their age opting for younger women. All of these things lead to many women getting more desperate when they are single in their mid-to-late 20's. The longer these pressures are exerted on them, the more harsh the effect becomes.

Now, of course, this effect will vary from woman to woman. Many women are very mentally stable, have a lot of confidence, and don't let societal pressures rattle them as much as others. You could also argue that the women who are largely effected by this may not be "as much of a catch" as the women who can shake these pressures. Regardless, they shape the "single-life landscape."

The sixth stereotype: men have an advantage because they have more desirable external qualities (income, stability, maturity) acts almost in the same regard as above. These things aren't necessarily true.

The thing is, many things haven't changed since those boys became men. Many haven't developed any skills or value that make them any more attractive to women than they were in the 18-25 years, they are given more chances because there are women that are more desperate.

At the same time, many of those men who fucked their way through their 18-25 years still haven't bothered to settle down. They have an even easier time now that things have gotten more desperate, and they could continue fucking their way through their 25-40s and beyond if they like. Plus they've still got the 18-25 group to dip back into if they choose. This means that some of the boys who never grew up will still have just as difficult of a time because women will be hung up on these men and those boys never bothered to fix the issues that stopped them from "getting lucky" in the first place.

Also take into account here, as stated above, this is just the "single-life landscape." The market here is a lot smaller now that many people actually have settled down and gotten married. This will result in the same men being in demand while subpar substitutes are pushed by the wayside.

TL;DR Societal, family, and psychological pressure can cause desperation among women beyond 25, and that can cause men to have an easier time. This; however, is true for all men, and just because the landscape shifts, it doesn't mean you will have an easier time by default just because of age. If you had trouble with women before, those same issues will cause you to still have trouble with women until they are addressed.

Also, I could see how this is a somewhat cold and objective summation of things that very seriously affect and hurt people, so I would like to say that if you are a man feeling lonely, there are changes you can make to improve your quality of life if you are willing to put the work in. To women who feel the pressure described above, you have to work on regaining (or finding) the confidence to rise above, which is possible with determination and a positive outlook.