OP, you shouldn't have any problems but like other posters suggested the parental consent letter may help you out of a tough situation! In Europe as long as you behave like an adult you'll be treated like one.
Originally Posted by
pinworm
As a person who is younger and going solo, you have a better chance of your seat being poached, or of an FA kicking you out of it to satisfy another passenger. When I was young they always targetted me for this. Stand your ground, ask to speak to the GA, and make sure that the seat they try to kick you to is of equal or better value.
+1, I still have to deal with this in my early 20s!