I don't think that's necessarily the thrust of the quote. It doesn't say "to remain youthful, with respect to the ability to unlearn falsehoods, requires unceasing cultivation of this ability". I don't know the context or the intent behind the quote, but it doesn't seem to imply for sure that young people generally have more of this ability than older people.
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 raili...
(Last month's started a little late, I thought I'd bring it back to its original schedule.)
A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.