Will God hold me accountable for masturbating at a young age without knowing it was forbidden?

If what you did was out of ignorance, then there is no sin upon you. And even if you deliberately committed the sin, the door of repentance is open, and Allah is Forgiving, Merciful. The one who repents from a sin is like one who has no sin. Take it easy on yourself, turn to your Lord, and strive in His worship.