Any age really. All players need a good coach! Children/youth players need a coach that listens and lets them learn their peers and by making mistakes. A good coach sets the framework for them and works on showing the player how they fit/ affect it. Get your qualifications and go for it , but remember football is a beautifully simple game that doesn’t need over complicating. Your players will have enough complications facing them when they are fully opposed by an opponent. Be calm, be patient and listen to your players when they talk to you.. good luck!!

There’s no best age to start. I started at u6 helping an established coach and learnt a lot by just being around and helping and doing bits of coaching.