Old people are told they are "young" when really what is meant is that they are healthy .
I think it can be rearrange to
When old people are told ( that ) they are "young" what is really meant is that they are healthy.
But I am not sure whether that is correct.
I tried to deconstruct it, I thought the core is:
What they are young is really meant is that they are healthy
But that does not seem to be right.