"I have found David...a man after My own heart." Acts 13:22 NKJV
David was anointed to be king in his teens, but he only ascended the throne at thirty.
There's a pattern here:
(1) God calls the man. In spite of being unappreciated by his family and overlooked by the prophet Samuel, God picked David. He also picked Deborah to lead the nation in a male-dominated society. Stop trying to figure God out, and stop comparing yourself to others! "Does not the potter have the right to make out of the same lump of clay some pottery for noble purposes and some for common use?" (Ro 9:21 NIV). We'd have rejected many of the people God used - which lets us know how much we don't know.
(2) God decides the plan. "Being confident of this very thing, that He who has begun a good work in you will complete it" (Php 1:6 NKJV). Who decides it? Who performs it? Who should your confidence be in? God! God's plan for David involved years of ducking Saul's spears, living in caves and working with 600 misfits who redefine the word "dysfunctional." God trains you in a small sphere so you can handle a bigger one.
(3) God knows the span of time required. "Who through faith and patience [endurance] inherit the promises" (See Col 1:11-12). Endurance means standing firm under pressure. The prize belongs to the man or woman who's committed for the long haul. Joseph refused to let go of his dream; it's what enabled him to say no to the advances of Potiphar's wife and endure unjust imprisonment. But the day came when Potiphar and his wife knelt before Joseph.
So keep your eyes on the prize and don't give up and stay on the path to the throne.