Translation:They are inviting the youth and adults.
It sounds more like a desire to become young again as opposed to 'young people', as a noun I would say the article the is required but even then would (without identifying WHICH 'youth', e.g. the youth of 'the council estate', 'the Jewish youth', etc.) in contemporary English use, more likely refer to a single person, a young lad, than your wish for it to refer to 'young people'. It sounds too archaic, with or without the article imho.