What to do about my parents and my older partner?

I am 21 years old, five months ago I met a man eighteen years older than me and we have developed what seems to me like a good relationship. My partner is 39 and has two children from a previous marriage, we get on brilliantly and to be honest no man has ever treated me with so much respect. I have told my friends about my partner and most of them are happy for me and don't feel there is anything wrong with it because although I may only be 21 they know I am mature. The problem is I feel as though it is time I tell my parents but feel that this could devastate them, my mum is a devout Catholic and dissaproves of the whole idea of divorce and my dad is very choosy when it comes to men I date, there is a ten year age gap between my parents but I still think this could upset them. I know I have to tell them as we are developing a serious relationship but i would like to do it causing as little hurt as possible. Any Ideas?