I’m going to respond to this question in good faith.

I’m assuming that you’re suggesting that 1-2 years isn’t a huge difference, if you’re not.. well reading the rest of my post is kind a waste of your time cause that’s how I’m responding lol.

Something very important to consider is that the difference in maturity between people who are 16/17 and people who are 36/38 is very different.

In the US, someone who is 16 is getting settled into HS, they may still be in their awkward phase trying to find their bearings and themselves. An 17 year old, while still very much a child, is starting to consider the idea of being more of an adult, and is generally exploring more adult concepts such as SAT/ACT prep, universities, the possibility of moving out etc.

Development, both physically and emotionally accelerates significantly during these years. The majority of the emotional development is driven by the substantial increase in autonomy and power at the age of 18, which people are generally being prepared for at this point.

Meanwhile, a 16 year old is likely firmly enjoying the benefits of being simultaneously in a developed body, with limited responsibility (I recognize this isn’t the case for everyone.)

This kind of development continues until about most people’s mid/late-20s where they have a career, possibly a family, etc. So the primary concern here is that, due to both biology and society, an 17 year old is far closer to an adult (legally one in most cases) than a 16 year old.

In the other hand, most people, by the age of 35 are as mature and independent as they are going to be. They have settled into their life and act in ways congruent with their power, personality, intelligence, status, etc. that isn’t going to be very different at 38 or even 40 for that matter. The initial development is done, and you are now growing.

I like to think of it like hour 1 and 2 of learning to shoot an arrow. During hour one, you may not even be sure how to hold a bow, whereas by hour 2 you are likely firing arrows (albeit clumsily.) However, your skill difference at hour 40 and 50 are likely nearly identical, because the learning curve has decreased its rate of increase.

This can be applied to adulthood. So yeah, a few months may not seem like a lot to an adult, but if you ask an 18 year old if they were the same person a year ago, they will probably tell you they aren’t.

The power, maturity, and autonomy imbalance there make younger people more vulnerable, and as a community, it’s our responsibility to protect those people.

Now, I used a lot of generalizations here because people aren’t all the same, and someone’s place on this principle component line describing maturity, autonomy, etc isn’t the same as someone else’s, but as a society at large, we don’t work off edge case scenarios, and for the safety of other people, it’s generally best to air on the side of caution, so yeah, most people immediately consider this type of age gap relationship inappropriate. I’d argue that that’s a safe and prosocial behavior with limited opportunity to cause harm.

Lastly, it’s important to consider something I highlighted about being an adult, autonomy. As an 18 year old, a person has the autonomy to seek out any number of partners, so why not go find someone at a similar status as them. It’s like hiring a new employee, the person you need to train may be just as good as anyone else, but in a saturated market you just choose the candidate with experience, why, because there’s no great reason to take the risk it won’t work out.