The Meaning of Numbers: The Number 90
The possible meaning of the number 90 is derived from both facts about and writings within Scripture.
When God changed Abram's name to Abraham (Genesis 17:5), and his wife's name Sarai to Sarah (Genesis 17:15), he also promised the couple they would bear a child. Abraham, at 99 years old, rejoiced at the promise by joyously laughing! He delighted in the miracle that would allow Sarah, who had already experienced menopause many years prior, to get pregnant at the age of 90 years young!
And Abraham fell upon his face and laughed, and said in his heart, "Shall a child be born to him that is a hundred years old? And shall Sarah, who is ninety (90) years old, bear?" (Genesis 17:17).
Appearances of the number ninety
Enos, the grandson of Adam, was born in 3734 B.C. He produced his firstborn son Cainan at the ripe old age of 90 in 3644 (Genesis 5:9)!
While on the subject of Enos, he belongs to one of the most exclusive clubs in the Old Testament. He is one of only seven humans known to have lived more than 900 (90 x 10) years. His death, at the age of 905 (Genesis 5:11), actually makes him the youngest of this rare club to perish! Those who lived a bit longer than Enos include his son Cainan (910 years, Genesis 5:14), his father Seth (912 years, Genesis 5:8) and his grandfather Adam (930 years, Genesis 5:3 - 5).
Rounding out the top three oldest humans in history, delineated in the Bible, are Noah (950 years, Genesis 9:29), Enos' great-grandson Jared (962 years, Genesis 5:20) and finally the number one oldest person named Methuselah (969 years, Genesis 5:27).
Moses and the number 90
Psalm 90 is the only Psalm that credits Moses (verse 1), during Israel's forty years of wandering the wilderness, as being the author. A few Biblical commentaries, however, call this credit into question.
The Psalm opens with asserting God's eternal existence (verse 2 of Psalm 90). It then bemoans the transience nature of human life by stating the best the average human can hope for is a scant 70 years of physical life. If, however, a person is healthy and strong, they may be lucky to live another 10 years and reach 80.
The days of our years are threescore years and ten; and if by reason of strength they are fourscore years, yet their span is but trouble and sorrow, for it is soon cut off and we fly away . . .
So teach us to number our days that we may apply our hearts to wisdom (verses 10, 12).
Moses was 80 when he confronted Egypt's Pharaoh (Exodus 7:7) and was 81 when the Israelites were punished by wandering the wilderness (Num. 13 - 14). He lived to be 120 years old (Deuteronomy 34:7). If he wrote Psalm 90, his own life would have contradicted what was written.
More Info on Biblical Meaning of 90
Psalm 90 also reveals the meaning of time from God's point of view.
For a thousand years in Your sight are as yesterday when it is past, and as a watch in the night. You carry them away as with a flood; they are as a sleep; in the morning they are like grass which grows up (Psalm 90 verses 4 - 5).
No chapter in God's word has exactly ninety verses and only one, Psalm 119, has more than this (176). It is also interesting to note that Genesis and Exodus, the first two books, combined have ninety chapters.