Answered by Shaykh Faraz Rabbani
In calculating missed prayers, from what age should one begin counting, if one is unsure as to when he/she hit puberty? I overheard some youngsters asking Mufti Ibrahim Desai at what age does prayer becomes obligatory for them, and his reply was simply “15.” And of course we are referring to Islamic years here aren’t we (i.e. 354 days vs. 365)?
The answer in related Q&A explains when one is legally considered to hit puberty. Use your conservative reasoned judgment about when you most likely reached puberty. In the absence of these signs, one would consider oneself to have hit puberty at 15 lunar (354 day) years, and make up one’s missed and invalid prayers accordingly.
Wassalam,
Faraz