What are the consequences for someone who started praying at the age of 18 and fasting at the age of 15, and how does one make up for what was missed, and at what age is a person held accountable for prayer and fasting?

If a child reaches puberty, they become responsible for religious obligations such as prayer and fasting. Puberty is indicated by one of three signs common to both males and females: emission of semen, growth of pubic hair, and reaching the age of fifteen. Women have two additional signs: menstruation and pregnancy. Whoever reaches puberty by one of these signs must make up for any missed prayers and fasts. Making up for prayers involves performing as many missed prayers as possible daily, in addition to the five current obligatory prayers, until one is reasonably certain that they have made up for what is due. When making up prayers, only the obligatory prayers are performed, not the supererogatory or sunnah prayers, unless the number of prayers is small. Fasting must also be made up if any days were missed after reaching puberty.