TL;DR I think that’s stupid.

Never heard such an argument before. There’s a distinction between what’s ethical and what’s legal, but age is merely an approximation for several factors: mental development, experience, the capacity for agency and consent, “life stage”, and various social/cultural nuances. It’s not practical nor feasible to define or test all of the possible metrics for every case: we must draw the line somewhere, and that somewhere is defined as a sufficiently developed brain and the (presumed) capacity for meaningful consent. Anything beyond that has exponential less significance… There’s a common saying: “half your age plus 7”; not that anything else is immediately unacceptable but that it may be suspect or deserving of greater scrutiny. Ultimately it doesn’t really matter so long as the meaningful consent requirement is satisfied. Unless someone can point to some data about significantly worse outcomes or harms, it’s my belief that age delta is largely irrelevant. And even if that were the case, it doesn’t automatically mean that such things should be restricted or socially punished (e.g. drinking and smoking causes harm but is permitted)