In the past, I lived among Sufis and committed some violations of Islam out of ignorance. However, I have repented, praise be to Allaah, but I did not have a ritual bath because I did not know that I was out of the fold of Islam then.
They used to tell us that a person is not responsible for acts committed before 18 years of age, which they fixed as the age of puberty, even if a person reaches puberty before that age. Based on this, I missed prayers while I was 15 and 16 and they did not guide me as they should have. Do I have to make up for the missed prayers?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

May Allaah The Almighty accept your repentance and help you to continue on the right path. However, you made a mistake by not taking a ritual bath soon after repentance while being aware that you had committed some actions that nullified Islam. The Prophet, sallallaahu ‘alayhi wa sallam, ordered some who had embraced Islam to take a bath, as in the case of Qays ibn ‘Aasim as narrated in At-Tirmithi and An-Nasaa'i, and the case of Thumaamah ibn Uthaal as narrated by Ahmad.

As for their claim that puberty starts at the age of 18, this is not agreed upon among scholars. According to Ash-Shaafi'i and Ahmad, the age of puberty is 15. Ash-Shaafi'i deduced evidence of this from the Prophet's, sallallaahu ‘alayhi wa sallam, rejection of `Abdullaah ibn Umar's recruitment for Jihaad while he was 14, and his subsequent acceptance of him when he was 15 and his father.

Apart from age, there are other signs of puberty. One of them is the discharge of semen. The Prophet, sallallaahu ‘alayhi wa sallam, ordered Mu‘aath to collect obligatory charity from all of those who had had wet dreams. [Abu Daawood]

Another sign is the growth of pubic hair. It is narrated on the authority of ‘Atiyyah Al-Qurathi that they used to kill male captives who had pubic hair as an indication of their puberty. [At-Tirmithi]

These signs mark the puberty of males; girls have two additional signs: menses and conception.

Moreover, non-observance of prayers is one of the major sins that takes a person out of the fold of Islam, if abandoned intentionally. It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that the separation point of disbelief is the non-observance of prayers. [Muslim, Abu Daawood, Ibn Maajah and At-Tirmithi]

It was also narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said that the person who abandons prayer is considered a disbeliever. [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa'i and Ibn Maajah]

According to Maalik and Ash-Shaafi'i anyone who does not pray out of laziness is to be asked to repent; if he still insists on not praying before the time of the current prayer elapses, he should be killed. Anyone who does not observe prayers by denying the obligation of this is a disbeliever according to the consensus of Muslims.

You do not have to make up for the prayers you missed before repentance, according to the predominant opinion of scholars. This is taken from the incident of a man who did not perform his prayers properly, and the Prophet, sallallaahu ‘alayhi wa sallam, ordered him to repeat the prayer that he saw him performing, and not all of his previously missed prayers. [Al-Bukhari and Muslim]

You should avoid evil places and everything that may lead to corruption of your religion. Ask Allaah The Almighty repeatedly to forgive you, do a lot of good deeds, and accompany righteous people so that Allaah The Almighty may accept your repentance. You may also refer to Fatwa 8500.

Allaah Knows best.