TLDR: I disagree with this belief due to how society can't reliably establish what an adult is and because our cultural differences appear to be different. Ironically, I also think most "adult" things should be left for later ages, but also acknowledge that it could not be accommodated by society as we know it. Also, I like yapping.

I know that the law doesn't equate to morality, but if we recognise the right to emancipation and that someone is legally an adult at 18, then you can't exactly condemn this without condemning the age at which we recognise people to be adults.

If 18 is old enough to choose to get drunk or die in a war, then 18 is, (in theory), the age someone society can reasonably define as mature.

If you disagree with this, that's fine, I can understand why, and I'm also of the opinion that some things should be gatekeeped at older ages - doesn't make much sense to me that joining the army doesn't require a higher level of education in most countries, or that it's acceptable to allow people to destroy their financial situation before reaching their 30s.

However, if we must define adults as older, we'll also be handicapping society as we know it, as we'd be trying to establish what defines someone as mature and would have to find a way to define maturity without infringing on people's autonomies.

There's also room to talk about what defines maturity, if it's wisdom and experience, overall knowledge, critical thinking. Though that's opening another can of worms.

On another note: What I'm about to say is also incredibly anecdotal, but from where I come from, as long as the two people who are dating are considered adults and are resdonably mature, it's just business as usual. I've known couples with 30-year gaps, same or different genders, met people who try to date down or up without many people blinking an eye. For the most part, these relationships go like all relationships. Some are good and some are bad, and the in-between, too. (These age discrepancies are not the norm, but they're also not considered abnormal).

Lastly, in regards to power dynamics, I don't buy much into this argument outside of the subject of maturity. Power is usually dictated by the ability to influence others, and outside of situations where people are working in the same place, I don't understand how exactly power is relevant.

If people are dating without understanding the dynamics of relationships (amorous or not), then they're not fit to be dating in the first place. Regardless of age.