The only context is that it appears in a set of other sayings of Lazarus Long in the interlude chapters of "Time Enough For Love", later collected into "The Notebooks of Lazarus Long". I've always thought it reasonable to assume that this is Heinlein himself talking. He had more of these aphorisms than could be worked into the dialogue.

Some people through the years accumulate more and more knowledge and beliefs, not all true, and never unlearn any of them. Whatever they acquire, they cling to, and end up as stiff, bitter old folks railing against a world they can no longer deal with. Others retain a lively intellect indefinitely, by always being open to the truth -- that is, to discovering that they were wrong. That is my interpretation of the quote.

As someone else put it:

"The things that we learn prevent us from learning."

-- W. Roy Whitten