In the novel It by Stephen King, a word which describes the tone in this excerpt best is nostalgic.
Nostalgic refers to the sentiments or affections which are related to the past, specifically for a place or periods relating to a happy personal association.
In the above excerpt line which depicts nostalgic affection is
“The energy you drew on so extravagantly when you were a kid, the energy you thought would never exhaust itself—that slipped away somewhere between eighteen and twenty-four,”
Richie gets nostalgic when he enters the room and feel the energy that he used to as a kid.