In reply to:

Definitions using "is when" are schoolbook examples of "primitive" or "infantile" talk over here.

And there's a lot that can be said in support of that point of view, wsieber. Yet, when wordminstrel quotes Thoreau, as I've pasted below, the quote doesn't sound either primitive or infantile to my ear. And I do wonder how many other instances exist of great writers having used the "is when" construction? It would be an interesting treasure hunt had I the time and inclincation to undertake such a search:

Thoreau didn't think so:
"Our truest life is when we are in dreams awake."~wordminstrel's observation and quote of Thoreau