Dang, I thought I'd already written a hub on this topic, but apparently haven't gotten around to it yet ;). My own husband is five years older than both of my parents, and a year younger than my stepdad. I also met him online while playing Myspace poker, and didn't meet him in person until the day he landed when he moved in with me. Suffice to say, my mom was not pleased when she heard about this match. That said, I knew that there was no way I could break the news to her in a way in which she would be pleased, or even be less shocked or concerned, so I treated it like removing a band-aid -- I came straight out and told her, then told her I'd be happy to discuss it with her once she'd calmed down and we could have a civilized conversation. Then I backed off and let her stew about it for a couple of weeks.
Eventually she calmed down enough to at least acknowledge that I am an adult and make my own decisions -- granted, he and I met when he was 49 and I 22, so that took her a bit of work to reach any sense of calm on the subject. Yes, she thought I was making a huge mistake and that my now-husband must be some kind of sick predator to want me. I think that's a reasonable enough reaction for a parent, especially given what few facts she knew. It was my own decision to let her have her differing feelings on the subject without trying to talk her out of them or let them bother me. She, in turn, reached a point where she stopped trying to control my relationship and maintained a civil demeanor regarding my husband. It took her the better part of three years before she accepted him as family, and started accepting that there are things to like about him and that he treats me well.
i agree, it's probably nice to warn your parent(s) ahead of time with a huge gap, but try not to insist on a positive, accepting reaction immediately. I'm glad that I told my mom before introducing her to my salt-and-pepper boyfriend who already struggles with presbyopia and has daughters older than me, but I'm equally glad that I simply acknowledged her protests and continued with the relationship anyway. He is the best thing that's ever happened to my life, we get along better than I ever could have imagined and have beautiful children together, and time has finally solidified Mom's faith in him too.