Is touching another person's penis at the age of fifteen, knowing that it is a sin with a lighter punishment than adultery, considered perversion, and can this sin be forgiven after repentance?

Not returning to the sin is not enough; repentance is obligatory, and its requirements are to cease the sin, feel remorse, and resolve not to return to it. If these conditions are met, then, God willing, the repentance will be accepted. What was done — despite being forbidden and contrary to human nature — does not reach the level of sodomy.