At our age it is not difficult to completely blend into western Europe. It will be difficult for you if you are not familiar with the culture, but after a few days you will be fine.
If you are interested in blending in and not being treated like a tourist, here are my tips. Bring your most fashionable wardrobe. Europeans take much more pride in their appearance than we do (at least in our age bracket, yes even Germans). Especially consider your footwear, almost no one wears the American style sneakers. However, if you are self conscious about it, just pick up a pair of kicks at one of the plethora of middle-eastern owned stores (super discount knock-off products and junk, but cheap, appropriate styles mostly).
Beyond your dress, your demeanor is the biggest thing for blending in. Just go with the flow and act like the local people are acting. Don't talk overly loud, and make ridiculous gestures at tourist spots and act like an American idiot and you will not be one. Also, you will soon see what an idiot American tourist looks like because they are everywhere in Europe and will be embarrassing to you.
Get of the tourist route as often as possible. While local store owners who do not cater to tourists may not speak English or have the best manners, they will treat you like a regular person until they realize you are American (that would be when it comes time to order and you point at a random selection or another diners plate with a silly grin and can't speak the local language). This is my favorite way to travel in Europe, the little local places are always the most interesting and in my experience have the best food and entertainment. If you do find yourself in a situation where you have little to no idea what you will be eating, just eat it with a smile on your face and be gracious to your host. I found that acting this way, made everyone in Europe (yes even the Parisians) like my wife and I and treated us better than their regular clientele.
Just be excited to be there and genuinely excited about the people culture and there is no way anyone will treat you badly.
Have the time of your life!!!!!!!!!!!!!!!!
p.s.- where are you going?