You could do it. If you were given that task, job or responsibility that is beyond anything you've yet done or beyond what others think you are old enough or experienced enough for - you could do it.
And you could even do it very well. But at what cost? It would be at the cost of fair few mistakes and a whole lot of learning from experience. And probably at a cost to your peaceful sleep and low stress levels.
Sometimes when God is saying "not yet", he's not actually questioning or doubting our ability. He's not saying "no way, you would be hopeless at that". He knows you could do it. He also knows what it would cost you. He also knows how much more satisfying and effective it can be when you take the time to prepare - mentally, physically, emotionally, spiritually.
He's saying "what I've got in store, you will enjoy it so much more if you wait until the right time, until it's not a stretch but a natural and right progression."
There is a reason Christians in ministry (or just life) burn out so often. It's not because God asks too much of us - it's because we take "I can do all things through Him who gives me strength" to mean "I Should do all things."
If God is saying "not yet", don't worry. He knows you could it. But funnily enough, God does actually want us to enjoy our lives and His plans, not just strain to reach higher and higher.
It's ok to wait.
Peace and joy. (Romans 5:1-11)
But God demonstrates his own love for us in this: While we were still sinners, Christ died for us.