I used to know a young lad who frequently travelled from his parents home, on his own, to visit his relatives in Paris. He was 12 when I got to know the family and he had already been making the journey regularly for a couple of years.
Most European countries have no specific laws relating to a minimum age for travel. (I think that Italy may have certain restrictions). In general, the only restrictions are those put in place by the various carriers. Most budget airlines, for example, won't carry unaccompanied persons under 16 years for insurance reasons.
Chris