He said the thing that makes the most sense for him to say. She wants to live her life, he wants her to live her life too. He wants her to grow older and feel free to fall in love, because he shouldn't be dictating her life.
However, the reason people
around him acted the way they did was because they both know and
admit to themselves that Mia already has fallen in love. That's why she wants to be a human. She fell in love with
him. It's the thing that makes this whole ticking clock thing compelling and tense, because all of a sudden what could be dismissed or lie dormant is something vital and immediate.
His potential arc in his mind for her could have happened. If she had always been going to school, if she had been in contact with regular people regularly, if she had learned to trust others and to see her life as something that she directed... maybe she would've actually fallen in love with one of those boyfriends she mentioned. Or maybe she would've met someone who could invigorate her the way Iori has. Who knows what could've happened, who knows how her life could've shaken out.
But it
didn't happen that way. Instead, she rolled through life certain that it would end soon and nothing really mattered. Until she met Iori. Who she
actually fell in love with, who made her want to see the rest of her life. That's the drama and tragedy in this supernatural romance, and why there's a gravitas to his decision here. Because things could've worked out differently. But they
didn't. They worked out the way they did.
I wrote a
post when the stakes were made clear, and I stand by it. It's what makes the manga interesting, because it's what makes the manga uncomfortable. Even the humor in this chapter comes from it. His honest, earnest, and totally reasonable thoughts fall on the completely deaf ears of the other catgirl, even with the previous reaction page. People who take it like 'ugh beta mc' or get mad at Tama are missing that the humor comes from the juxtaposition.
IIRC, the phrase used was one that literally translates as "around thirty" but is used colloquially for a guy a few years out of uni (i.e., not really an "onii-san" anymore) but not quite middle-aged (i.e., "oji-san"), either. He could be anywhere in the 25-35 range.
ADDENDUM: I'm amazed this is ending just as it's getting a NA release.
He's 27, it was first revealed when he met her brother.