I’m doing the same thing with three of my friends. We are all 18.

Another person has asked the same question in the past week. In hostels and all over, no ones cares about how old you are but many will care on how you act in public, trains, subways, tourist sites, bars, etc. Never talk about your ages, discuss more interesting topics. Hostels can have grade school kids on a school trip and also have people in their 50’s to 70’s trying to save money on their vacation during these tough times. Ages 17 – 28 will be the average. Be respectful of everyone you meet and go out of your way, say hello and meet people from all over the world to enhance your Eurotrip!
As for drinking, I have seen the discos in Picadilly, London become more strict when checking I.D. Drinking age is 18 and over. Don’t try a fake I.D. just walk in earlier when they need the business. They will be more picky as it gets crowded. Or just try smaller local pubs and most do not care but underage drinking is a concern of the owners due to increasing liabilty.
The Hofbrauhaus in Munich, Germany inforces a drinking age of 18 when it is 16 in the rest of the country. Mainly because of the 1-liter beers. You most likely will not have a problem in any other country. Just don’t get drunk in public. The alcohol is twice as strong as in the U.S.