do i have to make up for the prayer which i missed before i was 20

May Allah reward you abundantly in your effort to cover up for the missed Salaats, Aameen. You will have to start counting your missed Salaats from the time you attained puberty. (Maraaqiul Falaah pg.173)

1. When making up for your missed Salaat, the following intention should be made, e.g. Dhuhr Salaat ‘O Allah, from the many Dhuhr Salaats in my responsibility, I intend to perform the first Dhuhr Salaat I missed.’ In the similar manner, you make this intention for the other Salaats.

2. You will make up for your Fardh Salaats and Witr Salaats only. There is no need to make up for Sunnah and Nafl Salaats. (Aap ke Masaail vol.2 pg.348)

3. An easy method to cover up for the missed Salaats is that daily after every Fardh Salaat, cover up for one missed Salaat, e.g. after Dhuhr Salaat, read an extra four Rakaats for a missed Dhuhr Salaat.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai