In "Once More to the Lake," there is a reference to the chill of death in the last paragraph. What brings this feeling?

In this essay, E.B. White is talking about his feelings as
he and his son visit this lake that was a special place for White when he was young.  As
he visits, White sees himself in a number of different ways.  He sees himself as a youth
(through the eyes of his son).  He sees himself through his own eyes "now."  And he sees
himself as his own father who is now dead.

This is because
he is at this place where he went as a kid.  It reminds him of when he was young and so
he sees himself through his son's eyes.  At the same time, he is experiencing the lake
as his current self.  But he is also thinking about his father who had this same
experience long ago when White was young.  Thinking of his father, of course, reminds
him that he, too, will die.

So when White sees his son
doing things White himself once did, he thinks about how he was once young and is no
longer young.  He also thinks about how his father was once his age and is now dead. 
This brings White to think of his own death and that is why he feels the "chill of
death."