, at 66 , has stopped challenging himself 
(ROOT (S (, ,) (PP (IN at) (NP (CD 66))) (, ,) (VP (VBZ has) (VP (VBN stopped) (S (VP (VBG challenging) (NP (PRP himself))))))))
(ROOT (S (, ) (PP (IN ) (NP (CD ))) (, ) (VP (VBZ ) (VP (VBN ) (S (VP (VBG ) (NP (PRP ))))))))