Al-jawab billahi at-taufeeq (the answer with Allah’s guidance)
According to the Fiqhi Usool (fundamental principles)
“Al Waajibaatu Talzimul-Mukallifeen” (Obligations obligate the morally responsible i.e Buloog, attainment of maturity and ‘Aql, sanity).
To be Baligh, (to be physically matured) is one of the conditions that make Salah, Sawm and Hajj compulsory. In the case of the male, it is from when the signs of puberty appear, e.g. wet dream, growing of beard, etc. If there are no signs, then from the age of 15 lunar years.
In the case of a female, it begins at menses or if there is no menses then at 15 lunar years.
It is compulsory upon every Muslim male & female to perform the missed number of Farz and Waajib Salaat since the time one has become Baaligh (reached the age of puberty).
One should do Taubah (repent) before doing the Qadha Salahs, if one has missed Salah deliberately. If a person missed Salah due to reasons then one should just do the Qadha Salahs. (Ainul Hidayah Vol. 1 Pg. 574)
How to Perform Qadha Salah:
When making Niyyat for Qadha, It is necessary to make Niyyah for the particular Salah missed.
If one has missed a number of Salaat, then one should make Niyyat (intention) thus:
“I am performing such and such day’s Fajr or Zohar.”
If a person has missed more than one Fajr or Zohar, it will NOT be sufficient to say:
“I am performing Qadha for Fajr or Zohar.” One should say: “I am performing such and such day’s Fajr.”
If one has missed so many Farz Salaat that one does not remember the exact number of days when the Salaat was missed, then the Niyyat should be made as follows:
“Oh Allah! I am performing the first or the first Zohar Farz from those which I have missed.”
Continue doing this until satisfied that all the missed number of Salaat are performed.
And Only Allah Ta’ala Knows Best.
Moulana Qamruz Zaman