There is no absurdity, however palpable, which cannot be firmly implanted in the minds of all, if only one begins to inculcate it before the early age of six by constantly repeating it to them with an air of great solemnity.

[Es giebt keine Absurdität , die so handgreiflich wäre , daß man sie nicht allen Menden fest in den Kopf regen könnte, wenn man nur schon vor ihrem sechsten Jahre anfienge, sie ihnen einzuprägen, indem manunabläffig und mit feierlichstem Ernst sie ihnen vorsagte.]

Arthur Schopenhauer (1788-1860) German philosopher

Parerga and Paralipomena, Vol. 2, ch. 26 “Psychological Observations [Psychologische Bemerkungen],” § 344 (1851) [tr. Payne (1974)]
    (Source)

(Source (German)). Alternate translations:

There is no absurdity so palpable but that it may be firmly planted in the human head if you only begin to inculcate it before the age of five, by constantly repeating it with an air of great solemnity.
[tr. Saunders (1851)]

There is no absurdity, however palpable it may be, which may not be fixed in the minds of all men, if it is inculcated before they are six years old by continual and earnest repetition.
[tr. Dircks(1897)]