I don't think we're skirting it, I think your missing our point
Hypothetical
War is over, Tobias is still living as a hawk, but can morph his 13 yro body.
2.5 years later, Rachel is over 18, and they've been bonking
Tobias is also over 18.
His body is not, true.
But really there is no way to contest in court that he is 13.
His papers say he is 18
He can recount events over his life as an 18 yro would
And presently, if your arguing a person age in court, it's not a biological thing-it's a document thing.
They don't draw blood, assess it and say, he's 13, 18, whatever.
He's birth certificate says he's over 18. That's all they have.
You can legally have sex with someone over 18, even if for some biological reason their body is underdeveloped.
Courts would have to decide how morphing and aging is an issue, and draw up news ways to address it, but under current law I really don't see how this is a problem. Tobias is an 18 yro with a body of 13 yro- but he's still 18. If you want to look at it a certain way that's a bit kinky, but it's still legal