The article an is used in front of words that start with vowels but have a look at the following sentence:
England is a European country.
Why isn't it
England is an European country.
But Eu has a u sound....so why it shouldn't be England is an European country. Why should one consider Eu having the you sound rather than the u sound?
So as we say 'a young person', we also say a European'.
The letter u, the word 'you' and the 'Eu' in 'European' all have the long u sound. The short u sound is what the word 'uncle' starts with. So we say 'an uncle'.
You have to remember that the vowels have different long and short sounds. I hope it's clearer now.
long and short vowel sounds
long a make, short a cat
long e be, short e set
long i fine, short i skin
long o go, short o not
long u use, short u uncle