== ... but a... == 

 Disagree strongly with this  

 "He is but a youngster" (describing a middle-aged man) 

 I see that with the implied 'naught'  'He is naught but a youngster' -> 'He is nothing but a youngster' -> 'He is only a youngster'. 

 If the phrase were 'He is everything but young', then yes, agree that it would describe someone who is not young. 

 I've removed it from the article's list of examples pending an authoritative example or demonstration.