Whoso neglects learning in his youth, loses the past and is dead for the future.
Youth holds no society with grief.
Youth is the best time to be rich, and the best time to be poor.
A man loves the meat in his youth that he cannot endure in his age.
I go to school the youth to learn the future.
Age considers; youth ventures.
The young student sits with his head bent over his books, and his mind straying in youth's dreamland; where prose is prowling on the desk and poetry hiding in the heart.
Youth is wasted on the young.
So curiosity, I think, is a really important aspect of staying young or youthful.
Youthfulness is connected to the ability to see things new for the first time. So if your eyes still look at life with wonder, then you will seem young, even though you may not be chronologically young.