Is it obligatory for someone who abandoned prayer out of ignorance of its obligation and being misled by error to make up for those prayers, and is it permissible for him to perform supererogatory prayers before making up for them?

We hope that Allah accepts your repentance and keeps you steadfast. You erred by not performing ghusl (ritual bath) upon repentance, as the Prophet, peace and blessings be upon him, commanded some Muslims to perform ghusl. As for the age of puberty, it is not agreed upon that it is eighteen years; rather, Al-Shafi'i and Ahmad held that it is fifteen years. It also has other signs besides age, including: emission of semen, growth of coarse hair around the private parts, and for females, menstruation and pregnancy.

As for abandoning prayer, it is one of the greatest sins. Some scholars are of the opinion that whoever abandons it intentionally commits an act of disbelief (kufr). Whoever abandons it out of laziness is asked to repent; if he does not pray before the time for it expires, he is to be executed. As for whoever abandons it while denying its obligation, he is a disbeliever by the consensus of the Ummah.

As for making up the prayers you missed before repentance, the preponderant opinion is that you do not have to make them up. However, you must stay away from places of evil, increase in seeking forgiveness and performing all acts of obedience, and keep company with righteous people.