A fact of life = Any aspect of human existence that must be acknowledged, or regarded as unalterable (usually not pleasant)

• Old age is a fact of life.
• Stress is a fact of life from time to time for all of us.
• We should understand that death is a fact of life.

The facts of life = The facts concerning sex, reproduction, and birth (teaching of the same, exp., to a child)

• Some people feel that the facts of life should be taught in school.
• Raju learned the facts of life from his classmates.

‘The birds and the bees‘ is a euphemism(a mild or indirect word or expression) for ‘the facts of life’.

• It’s time Father told the children about the birds and the bees.
• She is so ignorant. She probably has not been taught so much as the birds and the bees.