I adore MitsuAi despite the big age gap, though it's a bit creepy, age doesn't have much to do with love.
My quick example, Kagome- 17 Inuyasha- 50. (Also my parents have a 10 year age gap, they're still married ^^)
Anyway, I support MitsuAi because, well, Ai-chan is more happy when she is with the Detective Boys.
If she were to change back to Shiho, what happiness does she have? She has no family or friends.
As much as I'd love to see her hang out with Ran, Kazuha, &Sonoko, I think she feels like she belongs in the Detective Boys.
She is happy as a child would be my point I'm trying to make.
So if she is happy as a child I don't think she'd mind growing up again and experiencing a simpler childhood.
Meaning, if she decides to stay as a child I'd say Mitsuhiko would be best with her. (Conan wishes to return to being Shinichi so that he can belong with his only true love RAN)
Mitsuhiko and Ai are compatible in ways. Mature &smart mainly. They both are related into the genre of science, no?
Ai also does seem to realize Mitsuhiko's feelings, and I'm sure she can accept them. (Not to mention flattered that a guy likes her that way)
So all in all, I think Ai will continue her life as a child and will eventually grow to like Mitsuhiko and accept him.
Just my opinion of course