That argument is such arrant bullshit you could use it as high-grade fertilizer.
They say fixed age laws don't take into account the emotional maturity of people, but that's simple: WE CAN'T DO THAT.
It is simply impossible to make a law covering a broad group without setting a discrete bar for when something is and is not okay. Humanity is not psychic, we cannot all sense when someone is "mature" enough for something, all we can do is set a fixed age when the overwhelming majority of us are expected to be physically and mentally mature based on a broad consensus.
It's not ideal, but the ideal is just not possible, so it's the best we got. Further, since we live in a world where some idiots and assholes try the "feels before reals" nonsense, if we gave them what they wanted, any immature person who said they felt otherwise would have to be catered to despite all good sense and reason, and fixed age laws are in force specifically to prevent that because the possibility of disaster is so high.
TL;DR: This is a "feels before reals" argument that needs pissed on and defiled because it's utterly naive at best and a wedge of deviants to get a loophole at worst.
