Upvote:4
As an under 18 you will not be allowed in some or even most hotels and hostels may have additional requirements.
The stories do differ, some young people report no problems at all, others report not being able to find hotels at all.
The additional requirements hostels seem to ask, when they do want them, is a letter of permission from your parents. The letter, mentioned in the answer by @Mark Johnson, will likely do for that. But if not mentioned, your parents may write a separate letter, just in English and/or German, would likely do.
If you can postpone your travel till you are 18 you will not have any problem with hostels or hotels.
It is not likely the people at the border will ask you for your letter of permissions, but you will feel more confident when you have it and that will show in your behavior and stance. Have proof of enough money (your bank card and a recent bank statement and if possible a credit card) in your hand luggage. If your parents are going to top up your bank account, having a letter of them with their promise is very handy to have.)
The biggest risk for young people traveling alone is that when things go wrong, they do not have the experience to handle it. And that experience does not only come with years but also with travel. If you have not (yet) traveled alone in your home country, you may want to do that instead or before coming to Europe. This is not legal but sensible advice.
Traveling with a commercial company or a group of people you know before setting out would be a better option than traveling alone.
Traveling with people you meet on the way or worse, you find online just for the reason of traveling together, has risks, especially if you trust each other with your money and passports. If you do end up traveling with people like that keep your own passport and only put a small amount of money in a common purse or pay your part each time or pay in turn and keep check you do not pay more or less than the other(s) so no bad things come from that.
Traveling alone can be lonely, be prepared for that and aware that it will become a problem.
It can be a good adventure. It can also turn into the worst nightmare you ever heard about. Be careful, even when a country is safe, it does not follow that all tourists in it are safe.
And trusting in people you meet while traveling can bring the nightmare about.
Upvote:10
It is advised that a minor (up to 18) travel with some form of consent from the parents and/or guardians.
Many automobile clubs offer standard forms, most in multiple languages.
The first page contains information about the minor, the second about the parent/guardian including ID or passport number
Some form of signature verification from a local authority is also advised.
Note: Some European countries require a signature authentication by their own Consulate, often using their own forms.
(based on information found on the German Foreign Office pages as of January 2020)
In Germany (and from the comments probably elsewhere) the signature and copy of the ID of both parents are needed (where applicable), otherwise it may not be accepted.
With this consent, booking a Hotel that would otherwise not allow a minor, is then possible in Germany and possibly elsewhere.
Sources: