I'm not Christian or an expert, but as I understand it, Biblically, there are some issues because a man "spilled his seed upon the ground" instead of inside his wife, but I think some people disagree over whether this means masturbation is sinful or whether it was the fact that he was engaging with his wife and pulled out that was the problem. Nowadays, I don't think many modern Churches have a problem with it (I don't know about Catholicism though... it might depend how strict it is).

In days gone by, marriage between young people made this less of an issue- when you got sexually able, you got a wife/husband, not your hand. In today's society, it might be that of two 'evils', the Church might prefer a person to take time with themselves than to be promiscuous, adulterous, etc.