Fifteen years and many worlds apart

And Hagar bore Abram a son, and Abram called the name of his son, whom Hagar bore, Ishmael. Abram was eighty-six years old when Hagar bore Ishmael to Abram.

Genesis 16:15-16 English Standard Version

So since Abraham was about 100 when Isaac was born, Ishmael was almost 15 by then. I'd never realized that before. I wonder why God promised Abraham billions of descendants 15 years before making it a reality--on the surface it would have seemed kinder/less confusing to either tell him this when he was young and newly married so that he and Sarah didn't panic when she couldn't get pregnant, or to tell him say, a week before she conceived or something.

Of course, God has a million unseen reasons on why He does anything, but I am awfully curious on this seemingly arbitrary timing.