When I was 14 year old I used to masturbate and I stopped. During that period and after doing this bad thing I didn't usually make the Taharah, so what should I do for my prayer (As-Salah) that I did without Taharah.

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

Dear Brother, you should not indulge in that bad practice. We ask Allah to forgive you and us.
Now, it is obligatory upon you to make up all those prayers that you performed without being in the state of Taharah (purity). You may perform them as convenient for you, i.e. you may pray with every obligatory prayer a missed prayer, or you may pray many missed prayers at any time of day or night.
We ask Allah to guide you to the good and the right way.
Allah knows best.