There's no fool like an old fool

But there is no foole to the olde foole, folke saie.
[1546 J. Heywood Dialogue of Proverbs ii. ii. F4^V]

No fool to an old Fool. Spoken when Men of advanc'd Age behave themselves, or talk youthfully, or wantonly.
[1721 J. Kelly Scottish Proverbs 256]

No Fool like the old Fool.
[1732 T. Fuller Gnomologia no. 3570]

And troth he might hae ta'en warning, but there's nae fule like an ould fule.
[1814 Scott Waverley III. xv.]

‘There are those who have years without knowledge.’ ‘Right,’ said Puck. ‘No fool like an old fool.’
[1910 R. Kipling Rewards & Fairies 257]

But these fantasies are more proper to a young person; beyond the age of, say, 50, they become the fantasy of that fool like whom we are told there is no other, the old fool.
[2001 Washington Post 8 July B5]

Related to: old age; stupidity

Bibliography of major proverb collections and works cited from modern editions is available here.

An old fool is the worst kind of fool, as in He's marrying a woman fifty years his junior--there's no fool like an old fool. This adage, now considered somewhat offensive for stereotyping old people, appeared in John Heywood's 1546 proverb collection and has been repeated ever since.