I would also say younger American tend to dress down, whereas mature European women tend to dress much more appropriately and... Dare I say it, more classily?
Almost everyone universally wears jeans, so a simple blouse that is classic would be non-noticable (unless you are wearing 3 sizes too small!) or a golf button down type shirt if you are a guy. Most Americans wears tshirts that have statements on then which are give aways.