The young person is an atheist because Christianity stands in the path of what makes him happy; the old person remains an atheist because Christianity offers eternal happiness - which he now despises as beneath him.
The first is an atheist because he is a hedonist who lives for pleasure; the second because he is a stoic who regards himself as advanced beyond any concern with pleasure.
The atheist youth is too proud to be bound by the conventional knowledge and petty rules of former generations and insists on doing it his own way; the atheist geriatric is too proud to admit the hope less ness of the life he has created for himself.
(The demons laugh at the triumph of pride. The transformation of a youth who will risk anything for the chance of pleasure to the elder who will sacrifice everything to reject the promise of happiness.)