I'm feeling some deja vu here except with a woman rather then a man. When Tom Hanks did this with Big, it could actually be funny at times and Tom Hanks pulled it off. With Jennifer Garner it was just embarrassing and completely unbelievable. Watching her trying to act like a 13 year old inside a 30 year olds body was beyond aggravating.
Basically, it's been done before and it shouldn't have been done again.