David was chosen to be king in his teens but only made it to the throne aged thirty. There are more lessons to learn:
(1) God calls the person. Despite 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. Let's stop trying to figure God out, and stop comparing ourselves to others! 'Doesn't a potter have the right to make a fancy bowl and a plain bowl out of the same lump of clay' (Romans 9:21 CEV)? (2) God decides the plan. 'He who has begun a good work in you will complete it' (Philippians 1:6 NKJV). Who began it? So who should our confidence be in? God! God's plan for David involved years of ducking Saul's spears and living in caves with misfits who redefine the word 'dysfunctional'. God trains us in a small sphere so we can handle a bigger one. (3) God knows the span of time required. Endurance means standing firm under pressure. The Bible says we inherit the promises through faith, endurance and patience (see Colossians 1:11-12). This means being like Joseph, who refused to let go of his dream; it's what helped him through all sorts of trouble, including years of unjust imprisonment. But the day came when a whole kingdom was handed to Joseph.
So what now? Keep your eyes on the 'throne' - your place in God's Kingdom - and don't give up! God is with you and he wants you to succeed.