What Is Youth?

I tend to save things that strike something in me that I want to think more about.  Sometimes I have an immediate thought or connection.  At other times, I want to explore it further.  Still others, I don't really know why...I like it, but I do.

The above, for example, makes me wonder what the essence of youth is.  Is it merely physical?  Is youth a thing of the mind?  Or, the heart?  Or, is youth a state of being - whether our body (or anything else) keeps up with it or not.  I'm guessing now, but I'm wondering if there is something about these people, aged as they appear to be, that is still quite young about them.

So, what is youth anyway?