Virgins going to war?

Isn't there some clause in the Pentateuch (the first five books of the bible) that makes a statement about virgins and them going to war or military service? I know there's a law giving exemption to newlyweds for their first year together, but is there something saying that 'you shouldn't goto war a virgin?' Or am I just a crazy 20 y/o trying to get some before I go play in the sandbox?