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.

Yes Yui is 17 in 2001.

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! :(