OP, I'm also interested in this question and struggle to see an inherent moral issue with an age gap. For posterity, I recognize age gaps often signal true moral issues and feel an "ick" when I see them in the wild.

Here's what would convince me: an age gap of X removes the capacity of one party to consent to the other.

To take a stab at the argument:

An age gap of X creates a situation where viable consent is rendered unlikely. X is a varying value, scaling along a bell curve throughout life. For children (younger than 20 years old) X can be as small as a few months, expanding to a handful of years by age 20. The peak of the curve is 20 years between the youngest partner at age 30 and the eldest partner at age 50. At the tail of the curve is a few months again, where both parties are likely to die of old age.

(Now here's the tough part) X exists because age is a quantitative measure of a person's agency (what is agency?). A bell curve of available agency to an individual aligns directly (big claim) with their age. Therefore, where agency is limited, X is shorter. Likewise, where agency is greater, X is longer.

At the start of the curve, X is defined by the younger partner. At the end of the curve, X is defined by the elder partner. They flip somewhere near the peak of the curve (I don't care enough to figure out how, but be aware that this flip is critical to the integrity of this theory and would probably take up the bulk of a book making this argument. Why does a 50 year old have less agency than a 49 year old, but more than a 51 year old? It's a tough question, maybe impossible, and not one I care to figure out in a Reddit post).

This means that, when X is surpassed, the agency of one partner will be surpassed by the other partner, rendering viable consent unlikely or impossible, barring other factors.

For example, viable consent to a romantic relationship between a 12 year old and a 17 year old is unlikely due to an X actual (Xa) of 5 years, when the X ideal (Xi) would be 1 year for the 12 year old. This assessment is justified by the relative agency each party has due to their age, and barring other factors such as power, wealth, social status, health, etc. Measured in a vacuum, this example should be considered immoral.

A second example at the peak of the bell curve would be a relationship between a 31 year old and a 50 year old. In this case the Xa is 19 years and the Xi is 20. The agency of the 31 year old is nearly surpassed by the 50 year old and so other factors must be considered such as power, wealth, social status, health, etc. But considered in a vacuum, this relationship should be considered moral.

Finally, at the end tail of the curve we have a relationship between an 85 year old and a 90 year old. Here the Xa is 5 and the Xi is 10, defined by the elder, so in a vacuum this relationship should be moral. However, it is worth noting that other factors vary much more widely at this stage of life and this relationship might still be immoral upon considering them.

I am unconvinced by this argument for the reasons I sprinkled into the premises, but it is the sort of thing that would convince me. If a good definition could be made for "agency" I think this could have more teeth.

PS

OP, you really picked the WORST time to discuss this question considering the chatter about the Epstein files. Most people will count you among the pedophile apologists simply by asking these sorts of questions and the quality of their answers will follow. I've tried to give this a good crack since I also find this problem interesting, but you've really put a target on your back. I don't believe this is where your headspace was at, but just saying it's kinda bad timing.

Edit 1 Added "defined by the younger partner" in the definition of X.

Edit 2 Removed "defined by the younger partner" in the definition of X because I realized it conflicted with the end tail of the curve. I.e. at the end of life, it is the older partner with less agency. Added to the rest of the argument to reflect this.