First you say it's okay because it's legal in your country, now you're saying it's about puberty. By the way, people are still going through puberty all the way through ages 16-18, so 14 still doesn't fit within your logic. If you can provide some kind of evidence that the "overwhemling majority" of people have finished puberty by 14, I'll concede on that.

Is it some ill-defined notion of mental maturity, for which 18 is unlikely to be a satisfactory age either?

So because it's not feasible to objectively draw the line on mental maturity, we should just outright ignore it? That's actually extremely fucking disturbing. You know full well that a 14-year-old is NEVER going to be nearly as mentally mature as an 18 or 21-year-old.

Actually, it's fucking disgusting that you acknowledge that it's possible for an 18-year-old to not be mentally mature enough to consent, but then defend 14-year-olds.

All of this is completely ignoring the concept of power dynamics and the role they play in consent. I suggest you look into it. There is no world in which it is possible for a grown adult with a job and responsibilities to engage in a truly consensual relationship with a 14-year-old that's still 4 years away from finishing their basic education.