Is the increase in the principal amount in my bank account – which I deposited since childhood and cannot withdraw until I reach 21 years of age – considered forbidden, necessitating its disposal, and is it permissible for me to use it now, or is it unlawful?

If you deposit money in a bank and it generates interest (riba), then you must withdraw the deposit as soon as you are able. There is no sin upon you before that time. The interest (riba) is unlawful wealth that must be disposed of by spending it on the welfare of Muslims or giving it to the poor and needy. It is not permissible for you to benefit from it unless you are poor and in need of it; in that case, you may take from it only what suffices your need. Your inability to withdraw the deposit absolves you of the sin of its continuation, but the interest remains unlawful. Inherited unlawful wealth does not become lawful. If you are in doubt whether inherited money is lawful or unlawful, it is lawful unless there is a sign indicating its unlawfulness. If you know that there is unlawful money within it but do not know its amount, then you should estimate and remove the unlawful portion through ijtihad (independent reasoning).