Great men are not always wise - Though wisdom may in general be looked for in them, yet it is not universally true. Great men here denote those who are distinguished for rank, age, authority.
Neither do the aged understand judgment - That is, they do not always understand it. The word judgment here means right, truth. They do not always understand what is the exact truth in regard to the divine administration. This is an apology for what he was about to say, and for the fact that one so young should speak. Of the truth of what he here said there could be no doubt, and hence, there was a propriety that one who was young should also be allowed to express his opinion on important subjects.