Personally, if I were you, I wouldn't tell them until I was in a situation where it was actually a viable possibility. By this I mean I'm old enough to be done with all my education, had been in a job long enough to earn decent money to fund the move, have a job lined up there, have done all the research into accommodation, visas, all the legalities, potentially made contacts out there to help the move go more smoothly etc etc.
I say this because regardless of how old you are, telling your parents you want to move country for whatever reason will probably ring alarm bells in their head. They'll obviously be aware of how much of a commitment it is, and that there are so many things you'll have to have sorted out. So if you tell them before all these things are in place, they'll likely discourage you or **** on the idea a bit. All for the simple reason that they care and wouldn't want you rushing into something which might not work out.
However if you've ticked the box for all the things I've mentioned and more, then when you break the news to them you'll be able to reassure them that everything is under control and they wont have any (or barely any) angle through which to discourage, which will make the whole process of telling them and having them understand and accept a lot easier.
You might also find it easier to break the news to them gently so rather than "I'm planning on going to america, this is the job I've lined up blah blah", it could be more of a "I'm thinking of one day moving to america, what do you think?" trying to get your parents input on it from the planning stage rather than simply informing them might ease the idea onto them. Involving them in the decision making process rather than telling them what you've done or planned would probably help both sides of this.
I have no idea how old you are, or your situation, but that's just how I'd focus on the breaking of news, rather than the reasons for going and if its a good/bad idea. Hope it helps somewhat.