I don't know, 15 boy an 18 year old girl sounds interesting if played right
Heck what if the two families were old dear friends an they made the agreement to marry but either lack of children on one part or the wrong gender
Then one day Born to his family so long awaited is born a boy
[IMG]http://th01.deviantart.net/fs71/PRE/i/2011/214/e/e/prince_caspian__the_tenth__by_wish_uponastar-d42fs97.jpg[/IMG]
The other family so long a waiting decides that this is their last chance and even though their youngest daughter is three years his senior they go for it