And the mental age is far more important.
But the problem is that he didn’t know initially. She was 12 he was 20, and he’s up and blushing. We know who she is and her mental age, but he didn’t. Otherwise he would have never let her get to that orphanage from the start of the story.
Normal people aren't attracted to kids even if they appear mature. It's not just the lack of mental maturity, the immature body is also a turn off.
Even knowing it's someone he loved he should be turned off by the body (which would make for a more interesting story since it could go two ways : 1-after he learn who she is and not a second before, the agony of loving her but having to wait for her to grow up because her body ruin his attraction and make him recoil every time they're romantic or 2-the evolution of his feelings, losing his romantic attraction for fatherly/sibling/familial love. Accepting that they're both different physically and mentally from the past and while love still remains it's not the same as before. A bitter-sweet ending where they naturally grow apart because their lives diverged.)
I don't expect teenage readers to understand tho, I sure didn't until I reached adulthood myself. As a teenager (or young adult as far as 22 year old) you can be attracted to adults and don't see yourself as a child so you don't necessarily understand why a healthy adult wouldn't be attracted to you.
I understand that, I was just specially addressing the mental age reply since the original commenter mentions the issue with her physical body. But I really would love to see a story with either of your two scenarios mentioned. Especially scenario 2, since in real life people grow and drift from each other all the time anyway (▰˘◡˘▰)
Like, what the FUCK, i know that before she died she's older than theo but that DOESN'T JUSTIFY why he romances a 12 YEAR OLD in her new body. Your conscience and physical age are completely and utterly different.