noun
BAD : There'll be a lot of youths at the party and so you should be able to enjoy yourself.
GOOD : There'll be a lot of young people at the party and so you should be able to enjoy yourself.
BAD : Life in a city is more interesting for the youth because there are more things to do.
GOOD : Life in a city is more interesting for young people because there are more things to do.
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Youth (countable) is used, often in a disapproving way, to refer to a boy/young man between the ages of about fifteen and twenty: 'He was attacked and robbed by a gang of youths.'
Youth (uncountable) is used mainly in formal styles to refer to all young people considered as a group in society: 'The youth of industrialized nations need to be made aware of global problems.'
The usual phrase for 'people between the ages of about fifteen and twenty' is young people : 'These holidays are designed for young people like yourselves.'