What are the necessary steps to declare my conversion to Islam, and is it true that it is not legally permissible before the age of 21?

If Allah guides a person to the true religion and opens their heart to Islam, then they have the same rights and obligations as other Muslims. It does not matter whether their conversion to Islam is publicly declared or not. Entering Islam is achieved by uttering the two testimonies of faith (Shahadatayn) with heartfelt affirmation and acting in accordance with their implications. If declaring one's Islam publicly is currently feasible, there is no need to delay, as it provides documentation and protection. If it is not feasible, there is no objection to postponing it for a valid reason (maslaha), while being diligent in performing the pillars and duties of Islam. There is no specific age in Islamic law for publicly declaring one's Islam. As for its determination by law, that is referred to legal experts.