My name is Madalina and I am 16. In June this year I am leaving my country by myself and have to fly in Germany as a stop-over to my final destination. I am interested in knowing whether there is a certain procedure or some sort of documents I have to hold in order to cross the border in and out of Germany, knowing I am underage.
There is no special procedure or any extra document needed for minors to cross the German border (you do need a passport/ID card and, possibly, a visa). Note that if you don't leave the international transit area, you probably won't have to go through a passport check at all and if you are an EU citizen, you should not be asked any question (or at least adults shouldn't).
While this is more of a concern for young children than for teenagers, border guards (or in fact the police, should you find yourself in some kind of trouble or unusual situation) might ask you a few questions to determine whether you have run away, been abducted or somehow removed from the custody of your legal guardians without their authorisation.
That's why having some letter from your parents or legal guardian can be helpful and is recommended by many sources including the German ministry of Foreign Affairs. It should at least be in English, possibly in German too and could even be notarised (as suggested by @Willeke and the ministry). But none of this is strictly speaking mandatory.
Some countries like France formerly required some sort of official document to allow minors to leave their territory without both their parents. As far as I know, this and other similar requirements have been abolished in most countries, certainly in the EU, and what's left are informal letters of the kind I described before. But do check whether your country has any rule like that as border guards there are more likely to be concerned about your leaving the territory than those in a transit country.
Incidentally, airlines have to check their passengers' travel documents and ensure they have the right to enter their destination and have transit visas as required. I don't think they will have a problem with your travelling alone but you might want to check with them too.
- have a valid passport
- are a citizen of a country that does not need a visa to land in Germany
- have an onward ticket to your final destination
- the right to land at your final destination
then you won't have any problems. Teenagers travel by themselves all the time. You can even have a beer at the airport bar. A beer. Singular. One.
This assumes that you are landing at, say, Frankfurt airport and just changing planes there. If you are staying overnight, or departing Germany by road or rail it can get a bit more complicated.
Madalina looks like a Romanian name, if you are in fact Romanian and your passport says "European Union" on the cover, then you can come right in. if German customs think you look a bit young your onward ticket should take care of any concerns they may have.
As far as I know, you need a signed and notarized letter by both your parents or all your legal guardians to cross borders while traveling under the age of 18. Until a better answer comes in, I would plan on getting one of those letters.
Added: Seeing the other answers, you do not need that letter to get into/through Germany but it is still a good thing to have on you when you travel. For younger children it might still be required. When the child travels with one of the parents as well as when the child travels alone.