At present, in modern american society, documents are everything. And for someone like Tobias, who's life story is known by everyone, it's not going to be contested what age he his.
There is no way to address the situation of a 18 yro in a child's body.
Age is defined as: The length of time that a person has lived or a thing has existed.
And the age of consent is depend on that definition, because it has no need, and no way to measure how that can contrast with a "body's" age.
So I don't think you have a way to say Tobias is not 18
Again, modern law doesn't address something that doesn't exist, such as this dilemma.
The closet we come are cases were a persons body fails or is slow to mature. They have the body of a child. But they are not. But if they've been on the planet 18 yrs they are legally adults. And they can do whatever they please with those bodies.
I don't know if there are laws for mentally disabled. But for almost all scenarios your age is from the year you were born, to the present, and if that's of legal status, you're good to go.
And a thought I've had before: It would seem odd, to me, near the end, were 4 16 yros are hanging with 2 13 yros. (Ax's human shouldn't age either)