I started observing Ramadhaan at age of 18 years. What can I do about all these years before? In what age Muslim must start fasting?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A person reaches the age of puberty by one of the following:

1)     Reaching sexual maturity, which is the ejaculation of sperm by a man or a woman while awake or in wet dreams.

2)     Growth of the pubic hair that requires to be shaved and not just the soft downy hair that covers the child's body.

3)     Reaching the age of 15 years.

In addition to this, there is a characteristic that is peculiar to women that is having menstruation.

In any case, since you started fasting after reaching the age of puberty, then it is an obligation upon you to make up for all the Ramadan months that you did not fast since reaching the age of puberty.  In addition to this you should feed a poor person for every day that you fast because of the delay, i.e. the delay in making up the fast at its due time, which is during the 11 months that are between one Ramadan and another.

Allah knows best.