So where do you draw the line? What if a 19 year old dates a mature 14-15 year old? Is that OK in your book, assuming theres jo significant power difference (if you agree its ok, explain your system of measuring 'power difference')?
Just because the specific number 18 is a social construct doesnt mean it should be a free for all. We do live in a society after all, and need to draw the line somewhere. Also, because in most countries (sans the US where you can drive at 16, go to war at 18, but only drink at 21) there is a definite line between adult and non-adult(the specific day you turn 18). This is necessary, even if it is a bit ridiculous that im not allowed to do certan things 24 hours before my 18th birthday.
In my country, and most if im not mistaken, if 2 underage people are dating, no one expects them to "pause" their relationship as soon as the older of the 2 crosses the threshold of adulthood, until the younger also crosses it. But if an adult starts a relationship with someone underage, thats not OK, abd you beed to have a cincrete definition of what 'underage' is, abd when exactly that is.