I don't remember the source, but i believe i read/heard somewhere (either in a game or the anime) the minimum age was lowered to 14 when yui was first dispatched.
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Copying from one is plagiarism.
Copying from more is research.
More Yui! the Better!
Sadly, since this thread is a bit old, most of the pictures have long since been deleted...how depressing to have only found out about Muv Luv a couple months ago thanks to a Hulu suggestion!