Also keep in mind that a 14 year old mother can´t offer the child as much as an adult mother. I think it´s sad to see so many smart babies who want to explore their enviroment and their teenage mommy doesn´t care about them, not offering them any attraction, or even a save and steady life. That however refers to modern times and not the Middle Ages.
In the pre-industrial society children were your life-insurance. They worked for you and only half of them made it to adulthood anyway. Many mothers died while giving birth and I bet a lot of them did so because they weren´t fully grown up. Even today a young mother will most likely need an "Imperial Cut" for everything to go well. So I also think there is more than religion behind that.