Formal:
* Na zawsze młody: This is the most literal translation and emphasizes the duration of youthfulness.
* Wiecznie młody: This translation emphasizes the ongoing nature of youthfulness.
Informal:
* Młody na zawsze: This is a more colloquial and playful expression.
* Zawsze młody: This is a more casual way to say "forever young."
Poetic:
* W młodości wiecznej: This translation is more poetic and emphasizes the timeless nature of youth.
The best translation will depend on the specific context and the tone you want to convey.