How long has God known you?  The Bible states that God saw you when you were a wrapped and unformed substance (embryo - a living thing in its earliest stages of development).

Psalm 139:16 "Your eyes saw my substance yet unformed and in your book they all were written, the days fashioned for me, when as yet there were none of them."

God also knew you before you were ever in your mother's womb.

Jeremiah 1:5 "Before I formed you in the womb I knew you..."

I believe that you have always been on God's mind.  "Even before the foundation of the world, Jesus Christ was the Lamb of God who was slain for our sins and forgiveness" (1 Peter 1:20).

"How precious are your thoughts to me o God!  How great is the sum of them!  If I could count them, they would be more in number than the sand"(Psalm 139: 17,18).