As everyone else has said, 19 isn't old at all. I have a bunch of friends who've started US uni at 19 and done fine.
Price-wise, however, is a different story. The best scholarships can be garnered from private companies (coca-cola, big 33, best buy, etc) and small, local scholarships, however, most of these aren't open to international students. Some schools offer merit scholarships (but none of those that you mentioned), but often, merit scholarships, too, aren't open to international students. I'd check out Washington University in St. Louis as a great school (6th or 7th in the country), as well as Emory University, Johns Hopkins University, and University of Chicago as more great schools with a bunch of merit scholarships that are available to int'l students. Yale, Brown, Stanford, Princeton, Harvard, etc are ridiculously expensive and, I warn you, just as selective as Oxford or Cambridge.
Schools like those (^) do offer need based aid and claim to meet "every bit of your need," but that's a load of crock. They'll want you to fill out a bunch of forms anyway (FAFSA, CSS Profile, etc) in order to be eligible for those, so bear that in mind, too. (The forms aren't that big of a deal, they just take FOREVER.)
Hope that helps somewhat!