Where ignorance is bliss
‘Tis folly to be wise.

Thomas Gray wrote these famous lines in his Ode on a Distant Prospect of Eton College.

Ignorance is bliss is a phrase frequently used on its own.  If someone is unaware of some problem or difficulty, they do not worry about it.  Bliss is a word which means perfect happiness or being in heaven.

There’s one born every minute.
This expression sounds vague, but is usually intended to apply to foolish people.

He’s not the sharpest knife in the drawer.
This is another euphemism for someone who is not at all bright.

Sharpen your wits.
It is possible to improve one’s mental agility by such activities as puzzle solving, playing games and learning new skills or using skills already acquired.

You’re sharp; have you slept in the knife box?
I used to think that a cutlery box was a knife box, but I have since been informed that a knife box used to be a means of sharpening knives.

Genius is an infinite capacity for taking pains.
This is a saying.  Taking pains is an old-fashioned way of paying attention to detail.

Genius is akin to madness is a saying expressed by John Dryden in Absalom and Achitophel.

Great wits are sure to madness near allied,
And thin partitions do their bounds divide.

Words of wisdom is a phrase which may be used I many ways, either seriously or jokingly.

None the wiser means in ignorance of a particular fact or facts.

Older and wiser is the condition many adults find themselves in compared with their childhood and youth.

Bowing to superior knowledge/wisdom means accepting someone else’s advice/scheme/opinion.

Losing one’s marbles is an expression about someone whose mental faculties are diminishing.  There is now a label for people with age-related mental problems such as confusion and memory-loss.  EMI stands for elderly mentally impaired.  What this has to do with either statues made from marble (like the Elgin marbles) or glass balls which are popular with children, I do not know.

You’re a rum un.
I have only heard this expression from northerners.  Rum means strange and un is dialect for one.

She’s all there with her cough drops.
All there is an expression meaning aware of one’s surroundings and mentally competent.  Perhaps such a person would be prepared for a tickle in the throat by having lozenges with them at all times!

The Bible has lots of information about wisdom and folly.  King Solomon was renowned for his wisdom and learning.  The Book of Proverbs was mostly written by him.  Proverbs 8 and Proverbs 9 are particularly relevant to this post.