Well I'm 20 and my boyfriend is 29, when I first told my folks I was going out with him that was when they first found out that he even existed... oh and that he lived in London and me in Ireland. So the shock of not knowing and the age difference I think sent them into a panic but at the end of the day they could see I was happy and had to trust me. They met my boyfriend very soon after and realised that he wasn't a rapist/perverted/paedophilic axe murder etc and have grown to like him very much! I'm telling you this because I recently read your post about the older guy so I figure thats maybe who you're talking about.
Anyway... my boyfriend being in London anall, the only way I could visit him was by travelling over there and staying for a weekend etc. I think it was about 2 weeks after they first met him that I went to visit him in London, I think I would have been 18 at the time. I'm sure, in fact I'm positive that they were uncomfortable about it, but... they have to learn to trust you. Technically you're an adult at 18 and although you probably shouldn't go throwing that in their face (since they already know its true), thats how it is. The best way is just to have a chat with them, tell them you know what your doing and that they have to let you grow up and to trust you.
My parents have admitted to being very weary at the start, but that they just needed time to adjust, they're only looking out for your best interests thats the main thing to remember...however unfair it may seem!
It'll all work out!