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