See the thing is, Mimi's age makes sense and is the main part of the story, not to mention she's mentally still a child of Sheena's age.
Certainly an
interesting rationalization.
Let's put a pin in that that for later.
Don't know that I'd say Mimi's age is "the main part of the story" though; her
immortality is the main part of the story, and the fact that has prevented her physically ageing is coincidental.
Here on the other hand Ooma (or whatever his name was) acts like a child while Riri is basically a grown woman, not to mention currently the age gap doesnt make any sense considering he died before Riri.
- Ooma is a girl; she refers to herself as such, as does everyone else, as does the narrative. Don't do weird misgendering.
- Saying she "acts like a child" seems very strange to say; she acts like a person.
It would take a real stretch to argue that simply being expressive with her feelings and intentions makes her child-like, when that's motivated by her past life's constant avoidance of the same.
- The narrative has yet to reveal any details about the reincarnation process; saying that it "doesn't make sense" relies upon making assumptions without evidence.
As long as the age gap turns out to be important to the story than I won't care anymore, as thats currently my main problem
Why do you believe there
is a problem?
They are two people, both of whom are
mentally adults - to borrow your justification of Mimi and Sheena, and it's not as though there's anything sexually explicit.
Not to mention that it already
is important to the story, because it's an inversion of their previous state.
Riri was formerly in a subordinate position, where she suppressed her own feelings and wants
because of their relative status and the strict social structure. Consequently, Ooma's avoidance and deflection determined the outcome.
(Something which Ooma herself has recognised was a very serious mistake, which she's actively working to not repeat.)
Post-reincarnation, their power dynamic is both diminished and inverted.
Their apparent ages are a
significantly lesser pressure than one of them being the sovereign ruler of the other, and any impact from the apparent gap will
necessarily diminish over time.
Which sets up for a very neat resolution with plenty of character beats along the way, including whatever secrets Riri is hiding.