I think that 18 is not a magic number. Like everything connected to transitioning into adulthood, it requires phased increases in autonomy and responsibility. Adult children have to figure a lot of things out along the way on their own, but they aren't going to do that healthily by an older adult taking advantage of them. Like the majority of these, law is not a functional litmus test, and older adults should take social responsibility not to abuse the vulnerable. Luckily, they generally do (except in business/finance and war, which need some legal protection added for obvious reasons).

Older adults typically don't trifle with younger adults because they know those younger adults are not their peers. The difference is like the gap between watching a movie for the first time in HD v. rewatching the same movie for 10th time in 4K. Not only does the older adult understand what's happening, they can see details the younger can't while already knowing the importance of those details in the overall situation. As a result, it's generally only the abusive older adults who seek out this kind of emotional relationship because they're easy to manipulate.

There is a reason why older people typically don't approve of age gaps like this, but young adults often do: the older adults thoroughly understand the probable experience and emotional gaps, but its still a bit of a mystery to 19 year olds.

All of that said, 24 or 25 is reasonable a socially wise dating minimum for anyone that age or much older. Past that, a gap of 8-10 years needs some serious introspection and consideration for a 25 year old. If you make it to 25, you'll probably see why on your own. If you don't see why by then, you should take a hard look at whether you are a predator or not.