We are exhorted not to despise someone's youth (1 Timothy 4:12; 1 John 2:14). In other words, we are not to be dismissive of young people whom God has called to teach. Understanding of truth comes from the Father (Matthew 16:16-17).
If we are young, we are also instructed not to despise the elderly but to learn from them (1Timothy 5:1-2; Titus 2:1-6).
God chooses whom he will use, how he will use and when he will use.
Do not doubt God's calling. Be obedient whether you are young or old.