All this "old man inside" bullshit is irritating.
OLD PEOPLE BONE
Even in nursing homes, "Old" people get down when they can.
Maybe the author was saying that you have more self control when you get older (which is true), but I doubt it; it is a firm belief of many, around the globe, that "old" people stop being interested in anything sexual, and Japan has a strong cultural belief in this...
And it simply is not true.
Old people Bone.
Now, if he feels like a Pedo by looking at her?
Okay, yeah; he would have no interest.
But... he's not actually "old," even if his memories are; he is a young, virile, human mage, who just experienced a life-or-death struggle, and she is plainly not physically 12 (I mean, unless she was born in 1973-1978; all the kids in late GenX had boobies/pube hair/active hormones by age 10, which was confusing for all of us), so...
Iunno; seems way beyond sus, to me...