Dare to be wise! Energy of spirit is needed to overcome the obstacles which indolence of nature as well as cowardice of heart oppose to our instruction.
by Friedrich von Schiller

Cultivate the habit of attention and try to gain opportunities to hear wise men and women talk. Indifference and inattention are two most dangerous monsters that you ever meet. Interest and attention will insure to you an education.
by Robert A. Millikan

Mingle a little folly with your wisdom; a little nonsense now and then is pleasant.
by Horace

[King] Solomon has made it clear that the need for preliminary studies is a necessity and that is impossible to attain true wisdom except after having been trained. For he says: If the iron be blunt, and he do not whet the edge, then must he put to more strength; but even more preparation is needed for wisdom.[Ecclesiastes 10:10] And he also says: Hear counsel and receive instruction that thou mayest be wise in thy latter end. [Proverbs 19:20].
by Moses Maimonides

Wisdom is the principal thing; therefore get wisdom; and with all thy getting get understanding.
From the Old Testament
Proverbs 4:7

You must not expect old heads upon young shoulders.
An English proverb