A wonderful quotation from JM Barrie's 1902 play "The Admirable Crichton", spoken by the character Ernest Woolley, who is a satire on the typical Wilde epigram-loving hero.
The quote is based on Wilde's "The old
believe everything; the middle-aged suspect everything; the young know
everything," from Phrases and Philosophies For the Use of the Young
A hat tip to an anonymous reader for providing the information above. The original (mis)information attributing the quote to Wilde came from Born to be Wilde, where the byline is a great quote from Wilde: "I don't want to go to heaven. None of my friends are there."