I would definitely carry the notarized letter. Whether you need it or not depends on the immigration officer(s) you encounter along the way, but I'd want to have it just in case.
I would also make sure you're in possession of a credit card. Or two different ones - say, a Visa and a Mastercard. Obviously it'll have to be a parent's account, but you may run into hotels or other travels that bristle at a debit card. Ideally a chip & sig card from a major U.S. bank. (I'm pretty sure you won't have chip & PIN like European banks do.) The chip isn't an absolute necessity though...just means some automated kiosks might not be able to read your card, so there may be a few train tickets and such where you need cash.
I backpacked Europe for a month when I was 20 and don't recall ever showing ID for age-related purposes anywhere. If you're 17 and look like a passable college freshman, I doubt you'll run into many issues. We stayed a hostels or the most bare-bones pensiones we could find. This was early 90's, so we were using a lot of cash, although even then I did carry 1 Visa and 1 MC with me, tied to accounts back in the U.S.
I also recall carrying a green student ID card - a general International Student ID, issued at our university travel agency in the student union. I think I paid about 10 pounds for it and it was good for museum discounts almost everywhere we went. Everybody seemed to have it...any box office that had a student discount knew what it looked like. Maybe U18's can just show a passport and get the same discount, but it might be worth looking into.
Agree with others who suggest obeying the local drinking laws.
