You are correct that people cannot help who they are attracted to.. regardless of age.
Let me start by defining some terms. For relationships, I believe that we find attraction and begin dating mostly for the sole purposes of monogamous companionship and child bearing. This is a social contract between two people to let their guards down so that they can exist completely with each others best interests in mind.
Maturity is based on the idea that different people have very different life experiences. Obviously you have some that I haven't had and there are some I have had that I would never wish for another person to have had. Your experiences influence your choices and the more experiences you have the more likely the choices you make will have the results that you intend them to have.
From the point of birth, you have the ability to make decisions. Every decision comes with a cost and those costs can be extremely dire. Ultimately you have to be responsible for your own choices. As a society, we have determined that a baby left to its own decisions will result more often than not in a dire way. So we pass that decision making on to the people who created the baby assuming they have the best interests of the child in mind. This creates the second social contract between two adults, to be willing to raise their children with the child's best interests in mind. In most cases this is to apply the most restrictive reasoning but not the adverse. So for example, I would expect the parent to tell the child who they cannot date but not who the child must date.
At some point we have to allow the individual to make their own decisions. The issue occurs with when? The parents in many cases would be happy to go on making those decisions until they die. This is not acceptable. So we have to base it on maturity. The problem with that, as we defined earlier, is that everyone has a different level of maturity based on their experiences. A child could have more maturity than their parents although this is less likely just due to total time of existence. So we say by certain ages a child should have had enough experiences to be able to decide this or that for themselves. In most cases we define that point to be the age of 18.
At 18 are the people old enough to be responsible for the results of their choices? The answer is some yes and some no. Without someone to make that decision for every single person, we are left using a default age and hoping for the best. Still many people's experience with 18 year olds make them wonder how prepared they really are for this. We coddle them more and more and as such we keep them safer but it becomes more difficult at 18, 19, 20, 21 to become responsible for their decisions.
So yes, I agree we have no choice who we are attracted to. Yes I think in many cases it is wrong to just perceive the decisions of an 18 year old as gross. I do, however, understand their position. They only want the best decisions for the 18 year old. They don't know the current experience of the 18 year olds. They only make decisions based on their own experiences which could be drastically different. The fact is that at 18 they are responsible for their decision making. They always have been but now is when we decide their parents shouldn't be allowed to stop it. Everyone walks a different path and people should be able to review that person's path individually. Instead of saying eww it should be I hope that path is going well for them. If that person asks for advice, we should review the relationship as any other relationship and determine if the advice is sound advice regardless of the age.
Age gaps bring a new set of issues. Those issues should be attributed to the age gap. Ones that don't apply to the gap should be reviewed honestly without the gaps influence.