The argument is usually presented like this:

1. The prefrontal cortex, which is heavily involved in the decision-making processes of our brains, doesn't fully develop in humans until about 25 years of age.
2. Because of this, women below the age of 25 are not in a position to make good judgements about the men they date.
3. And because of this, older men should not be dating these younger women and exploiting this sub-optimal decision-making.

If we accept both the premises and the conclusion of the argument, we are left with two possibilities and only two possibilities:

1. Women under 25 should not be dating anyone, at all.
2. Women under 25 should not be dating men over 25.\*

*Close-in-age exemptions would not work here (like a 24 year old woman dating a 26 year old man) because the argument is that there is something fixed around this age that is substantial enough to make any relationships with a girl below this age potentially predatory. If there indeed is something substantial enough about this age, then any man over 25 would be capable of exploiting this sub-optimal decision making on the part of the woman. If there isn't actually something substantial enough about this age then that would mean the original argument falls apart. (And rather than demonizing all age gap relationships, we would have to look at them on a case-by-case basis on the particular maturity of the individual woman and the good/ill intentions of the individual man.)

To get to the propositions, though, it must be said that the first proposition is absurd. Girls hit puberty around age 11. Asking them to hold off dating for 13 years, at the peak of their beauty and fertility, is contrary to reason. In fact, a female's fertility is only about 34 years total (ages 11-45). By asking them to remain single until 25, you're asking them to remain single for 38% of their fertile years (and like I said, these aren't just any fertile years, they are a woman's peak fertile years).

The second proposition is equally absurd. If the only men available for these women to date are men also under 25, that would mean you have a situation in which women with sub-optimal decision-making capabilities are only allowed to date men with sub-optimal decision-making capabilities.

To put it another way, you are forced into the corner of having to defend two sub-optimal decision-makers dating each other as superior to a person with sub-optimal decision-making dating a person with optimal decision-making. "I think it's better for your well-being, dear, that you date someone who is as bad at making decisions as you are." It's absurd, especially if the true motive behind all of this is to protect women.

But all of this rests on the first and second premises of the original argument being true: that the prefrontal cortex doesn't fully develop until 25, and until this develops, women are not in a position to make optimal decisions. Are these things true?

New research from the UK shows the brain continues to develop after childhood and puberty, and is not fully developed until people are well into their 30s and 40s. The findings contradict current theories that the brain matures much earlier.

https://medicalxpress.com/news/2010-12-brain-fully-mature-30s-40s.html

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The part of the brain that keeps growing is the prefrontal cortex.

This area of the brain – which is located at the front of the head - is involved in a host of important factors – including attention, complex planning, decision making, impulse control, logical thinking, organised thinking, personality development, risk management and short term memory.

But because the brain keeps on changing, even saying that change has largely levelled off by 30 is a problem.

Dr Somerville said that as the brain keeps changing throughout adult life, this ‘challenges the very notion that the brain reaches a steady adult referent that we can concretely call “mature”.

https://www.dailymail.co.uk/sciencetech/article-4055490/You-think-grown-18-brains-don-t-fully-mature-hit-30.html

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White matter volume peaks around age 40.

Much of the added white matter represents increased connections between widely separated brain areas. During childhood and adolescence, most brain networks are locally organized — areas near each other work together to accomplish a cognitive task. As we mature, distant areas of the brain begin linking up with each other, leading to larger and more widely distributed networks.

The most important brain area to become fully “wired up” in adulthood is the prefrontal cortex...

... While the adult brain is more cognitively capable than the teen brain, there might not be a single point in adulthood at which all (or even most) of our cognitive functions operate at peak performance.

https://www.brainfacts.org/thinking-sensing-and-behaving/aging/2018/when-the-brain-starts-adulting-112018

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"By the age of 15 years, there is little difference in adolescents’ and adults’ decision-making patterns pertaining to hypothetical situations. Teens were found to be capable of reasoning about the possible harm or benefits of different courses of action; however, in the real world, teens still engaged in dangerous behaviors, despite understanding the risks involved. Hence, both the role of emotions and the connection between feeling and thinking need to be considered while studying the way teens make decisions."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3621648/

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Adolescents do about as well as adults on cognition tests, for instance. But if they’re feeling strong emotions, those scores can plummet. The problem seems to be that teenagers have not yet developed a strong brain system that keeps emotions under control.

Dr. Steinberg agreed with Dr. Somerville that the maturing of the brain was proving to be a long, complicated process without obvious milestones. Nevertheless, he thinks recent studies hold some important lessons for policy makers.

He has proposed, for example, that the voting age be lowered to 16. “Sixteen-year-olds are just as good at logical reasoning as older people are,” Dr. Steinberg said."

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"It’s not enough to compare people using simple categories, such as labeling people below age 18 as children and those older as adults. “Nothing magical occurs at that age,” Dr. Somerville said."

https://www.nytimes.com/2016/12/21/science/youre-an-adult-your-brain-not-so-much.html

Obviously something is happening around the age of 24 in the development of brains, but it's a lot messier and a lot more fluid than treating pre-25 women as if they are basically children in adult bodies. And if you do decide to do that (and decide that, consequently, older men shouldn't date younger women for this reason), you are forced into picking one of the two absurd propositions mentioned earlier. The solution is simple: men (and women for that matter) should obey the just laws of the land they live in, and not pay much attention to shaming tactics that are based on unconvincing logic and scientific data that is a bit scattered and quite individual.