Jugend vs. Jugendzeit
The German words Jugend, Jugendlichkeit, and Jugendzeit all revolve around the concept of youth. However, they are not interchangeable, as each carries a distinct nuance. Jugend is the most general term, Jugendzeit specifically refers to the time period, and Jugendlichkeit describes the quality of being youthful.
Jugend
A2Jugend is the most common and versatile word. It has two main meanings: 1. The period of life between childhood and adulthood. 2. Young people as a collective group.
In seiner Jugend war er sehr sportlich.
(In his youth, he was very athletic.)
Die Politik muss die Sorgen der Jugend ernst nehmen.
(Politics must take the concerns of the youth seriously.)
Sie hatte eine unbeschwerte Jugend auf dem Land.
(She had a carefree youth in the countryside.)
Dieses Festival richtet sich an die Jugend.
(This festival is aimed at the youth.)
Jugendzeit
B1Jugendzeit specifically emphasizes the time period or era of one's youth. It is a compound word (Jugend + Zeit meaning time) and is often used when looking back reflectively or nostalgically at that specific phase of life. It cannot refer to young people as a group.
Er denkt oft an seine Jugendzeit in den 90er Jahren zurück.
(He often thinks back to the time of his youth in the 90s.)
Während meiner Jugendzeit gab es noch keine Smartphones.
(During the time of my youth, there were no smartphones yet.)
Ihre Jugendzeit war von vielen Reisen und Abenteuern geprägt.
(The time of her youth was marked by many travels and adventures.)
Das Fotoalbum ist eine schöne Erinnerung an unsere Jugendzeit.
(The photo album is a beautiful memory of our time as youths.)
Summary
In short: use Jugend for the general concept of youth or for young people as a group. Use Jugendlichkeit to describe the attribute or quality of being youthful, regardless of actual age. Use Jugendzeit to specifically refer to the time period of one's own youth, often in a nostalgic context.