If it's a serious relationship you are going to want to tell her sooner or later. why wouldn't you. Everyone is so gung ho about coming out, but people here are advocating deception and lying in this situation? Double standards, sorry.
It's probably going to take some adjusting and TIME. Even if you were dating a 60yo woman it would take a lot of adjustment for your parents to get used to. But the more they see he is not taking advantage of you, which would probably be their main concern, and that it is a real and loving relationship they will get used to it and grow to accept it. it's no different to them adjusting to any other kind of relationship they hadn't imagined for you. Let your parents have their little temper tantrums and shock and awe about it, it's a natural reaction, they will get over it eventually, and if they don't it's their loss at least you were honest and didn't try to keep it a secret from them which I think would feel more hurtful to a parent than their child finding love, in whatever form they find it.